Choose the Correct Illumination

Often, a customer struggles with contrast and resolution problems in an imaging system, while underestimating the power of proper illumination. In fact, the desired image quality can typically be met by improving a system’s illumination rather than investing in higher resolution detectors, imaging lenses, and software. The system integrators should remember that proper light intensity in the final image is directly dependent upon component selection.

Correct illumination is very critical to an image system and improper illumination can cause a variety of image problems. Blooming or hot spots, for example, can hide important image information, as can shadowing. In addition, shadowing can also cause false edge calculations when measuring, resulting in inaccurate measurements. Poor illumination can also result in a low signal-to-noise ratio. Non-uniform lighting, in particular, can harm signal-to-noise ratios and make tasks such as thresholding more difficult. These are only a few of the reasons why correct illumination for your application is so important.

The pitfalls of improper illumination are clear, but how are they avoided? To ensure optimal illumination when integrating a system, it is important to recognize the role that choosing the right components plays. Every component affects the amount of light incident on the sensor and, therefore, the system’s image quality. The imaging lens’ aperture (f/#) impacts the amount of light incident on the camera. Illumination should be increased as the lens aperture is closed (i.e. higher f/#). High power lenses usually require more illumination, as smaller areas viewed reflect less light back into the lens. The camera’s minimum sensitivity is also important in determining the minimum amount of light required in the system. In addition, camera settings such as gain, shutter speed, etc., affect the sensor’s sensitivity. Fiber optic illumination usually involves an illuminator and light guide, each of which should be integrated to optimize lighting at the object.

Table 1: Key Photometric Units
1 footcandle = 1 lumen/ft2
1 footcandle = 10.764 meter candles
1 footcandle = 10.764 lux
1 candle = 1 lumen/steradian
1 candle = 3.142 x 10-4 Lambert
1 Lambert = 2.054 candle/in2
1 lux = meter candle
1 lux = 0.0929 footcandle
1 meter candle = 1 lumen/m2

The light intensity for our illumination products is typically specified in terms of footcandles (English unit). Lux, the SI unit equivalent, can be related to footcandles as follows: 1 lux = 0.0929 footcandle.

Table 2: Illumination Comparison
Application Requirement Object Under Inspection Suggested Type of Illumination
Reduction of specularity Shiny object Diffuse front, diffuse axial, polarizing
Even illumination of object Any type of object Diffuse front, diffuse axial, ring light
Highlight surface defects or topology Nearly flat (2-D) object Single directional, structured light
Highlight texture of object with shadows Any type of object Directional, structured light
Reduce shadows Object with protrusions, 3-D object Diffuse front, diffuse axial, ring light
Highlight defects within object Transparent object Darkfield
Silhouetting object Any type of object Backlighting
3-D shape profiling of object Object with protrusions, 3-D object Structured light

Types of Illumination :-

Since proper illumination is often the determining factor between a system’s success and failure, many specific products and techniques have been developed to overcome the most common lighting obstacles. The target used throughout this section was developed to demonstrate the strengths and weaknesses of these various lighting schemes for a variety of object features. The grooves, colors, surface deformations, and specular areas on the target represent some of the common trouble areas that may demand special attention in actual applications.

 fig-1-cci

Directional Illumination – Point source illumination from single or multiple sources. Lenses can be used to focus or spread out illumination.

Pros

Bright, flexible, and can be used in various applications. Easily fit into different packaging.

Cons

Shadowing and glare.

Useful Products

Fiber optic light guides, focusing assemblies, LED spot lights, and incandescent light.

Application

Inspection and measurement of matte and flat objects.

Directional Illumination

Directional Illumination2

Glancing Illumination – Point source illumination similar to directional illumination, except at a sharp angle of incidence.

Pros

Shows surface structure and enhances object topography.

Cons Hot spots and extreme shadowing.
Useful Products Fiber optic light guides, focusing assemblies, LED spot lights, and incandescent light and line light guides.
Application

Identifying defects in an object with depth and examining finish of opaque objects.

Glancing Illumination

Glancing Illumination2

Diffuse Illumination – Diffuse, even light from an extended source.

Pros

Reduces glare and provides even illumination.
Cons Large and difficult to fit in confined spaces.
Useful Products Fluorescent linear lights.
Application

Best for imaging large, shiny objects with large working distances.

Diffuse Illumination

Diffuse Illumination2

Ring Light – Coaxial illumination that mounts directly on a lens.

Pros

Mounts directly to lens and reduces shadowing. Uniform illumination when used at proper distances.
Cons Circular glare pattern from reflective surfaces. Works only in relatively short working distances.
Useful Products

Fiber optic ring light guides and fluorescent ring lights; LED ring lights.

Application

Wide variety of inspection and measurement systems with matte objects.

Ring Light

Ring Light2

Diffuse Axial Illumination – Diffuse light in-line with the optics. Lens looks through a beamsplitter that is reflecting light onto the object. Illumination is coaxial to imaging access.

Pros

Very even and diffuse; greatly reduces shadowing; very little glare.

Cons

Large and difficult to mount; limited working distance; low throughput such that multiple fiber optic sources may be needed to provide sufficient illumination.

Useful Products

Fiber optic diffuse axial attachment. Single or multiple fiber optic illuminators. Single, dual, or quad fiber bundles depending on size of attachment and number of illuminators used. LED diffuse axial illuminator.

Application

Measurements and inspection of shiny objects.

Diffuse Axial Illumination

Diffuse Axial Illumination2

Structured Light (Line Generators) – Patterns that are projected onto the object. Typically laser projected lines, spots, grids, or circles.

Pros

Enhances surface features by providing intense illumination over a small area. Can be used to get depth information from object.

Cons

May cause blooming and is absorbed by some colors.

Useful Products

Lasers with line generating or diffractive pattern generating optics.

Application

Inspection of three-dimensional objects for missing features. Topography measurements.

Structured Light

Structured Light2

Polarized Light – A type of directional illumination that makes use of polarized light to remove specularities and hot spots.

Pros

Provides even illumination over the entire surface of the object under polarization. Reduces glare to make surface features discernable.

Cons

Overall intensity of light is reduced after polarization filter is placed in front of light source and/or imaging lens.

Useful Products

Polarization filters and Polarizer/ Analyzer adapters.

Application

Measurements and inspection of shiny objects.

Polarized Light

Polarized Light2

Darkfield – Light enters a transparent or translucent object through the edges perpendicular to the lens.

Pros

High contrast of internal and surface details. Enhances scratches, cracks, and bubbles in clear objects.

Cons

Poor edge contrast. Not useful for opaque objects.

Useful Products

Fiber optic darkfield attachment, line light guides, and laser line generators.

Application

Glass and plastic inspection.

Darkfield

Darkfield2

Brightfield/Backlight – Object is lit from behind. Used to silhouette opaque objects or for imaging through transparent objects.

Pros

High contrast for edge detection.

Cons

Eliminates surface detail.

Useful Products

Fiber optic backlights and LED backlights.

Application

Targets and test patterns, edge detection, measurement of opaque objects and sorting of translucent colored objects.

Backlight

Backlight2

Filtering Provides Various Levels of Contrast

Examples illustrate darkfield and backlight illumination with assorted color filters. Note: Images taken with 10X Close Focus Zoom Lens #54-363: Field of View = 30mm, Working Distance = 200mm.

fig-11a-cciDarkfield Only Defects appear white

fig-11b-cciDarkfield with Blue Filter Defects appear blue

fig-11c-cciDarkfield and Backlight No filter used, but edge contrast improves

fig-11d-cciDarkfield without Filter and Backlight with Yellow Filter Enhances overall contrast, defects appear white in contrast to rest of field

Image Enhancement using Polarizers

A polarizer is useful for eliminating specular reflections (glare) and bringing out surface defects in an image. A polarizer can be mounted either on the light source, on the video lens, or on both depending upon the object under inspection. When two polarizers are used, one on the illumination source and one on the video lens, their polarization axes must be oriented perpendicular to each other. The following are polarization solutions to glare problems for several material types and circumstances.

Problem 1:-

The object is non-metallic and illumination strikes it at a sharp angle.

Solution:-

A polarizer on the lens is usually sufficient for blocking glare. (Rotate the polarizer until glare is at a minimum.) Add a polarizer in front of the light source if glare is still present.

fig-12a-cciWithout Polarizers

fig-12b-cciUsing Polarizers

Problem 2:-

The object has a metallic or shiny surface.

Solution:-

Mounting a polarizer on the light source as well as on the lens is recommended for enhancing contrast and bringing out surface details. The polarized light incident on the shiny surface will remain polarized when it’s reflected. Surface defects in the metal will alter the polarization of the reflected light. Turning the polarizer on the lens so its polarization axis is perpendicular to that of the illumination source will reduce the glare and make scratches and digs in the surface visible.

fig-13a-cciWithout Polarizers

fig-13b-cciUsing Polarizers

Problem 3:-

The object has both highly reflective and diffuse areas.

Solution:-

Using two polarizers with perpendicular orientation will eliminate hot spots in the image caused by the metallic parts. The rest of the field will be evenly illuminated due to the diffuse areas reflecting randomly polarized light to the lens.

fig-14a-cciWithout Polarizers

fig-14b-cciUsing Polarizers

Article is posted by Optics For Hire – lens design and manufacturing Consultant.

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Build Your Own Light Pipe

A Step-by-Step Guide to Build Your Own Light Pipe: Light guides are physical tools that transmit light along a path from an illumination source while maintaining constant brightness. They’re often used in products to create a line of light that is consistently bright and dynamic-looking, even when it comes from a single source. Unfortunately, there is no simple how-to guide available for making them. The process is kept tightly guarded by optical engineering companies. You can follow a quick process to make your own light guide.

We built a light pipe, which, in principle, can be viewed as a fiber optic cable. All the sides are polished to a clear finish so that the light can be reflected internally inside the pipe. If the light hits a section of pipe where the surface is not perfectly reflective, like if it’s gouged or scratched, it will use that section as an “emitter” and attempt to escape the pipe. We take advantage of this by covering the bottom surface of the pipe with dimples. To make the path consistently bright, we arranged the dimples in a gradient with fewer dimples closer to the source, where the light from LEDs is brightest, and more dimples to catch the light farther along the light path (where the light is dimmest).

This step-by-step guide describes our process of quick experimentation and prototyping to achieve good illumination without having to run extensive light bouncing simulations. The design is created in Illustrator using blend modes, and the light pipe is manufactured with a laser cutter on 1/4″ thick clear acrylic. Please note that results will vary depending on many factors. The optical design, LED brightness, and beam angle are important factors to consider.

1: Open a new Illustrator document and draw a rectangle. We chose 8″ x .5″.

2: Make a guide and space it roughly 1/4″ away from the edge of the rectangle. This is where the dimple pattern will begin. (Leaving a 1/4” margin allows us to later mask off the area to prevent seeing excessive LED brightness near the source in the light pipe.) Now draw a circle with a .005″ diameter.

3: ALT + Click and drag the circle to duplicate it, and move it to the other side of the rectangle. The two circles mark the two ends of the dimple pattern.

4: Select both circles and create a blend.

5: Open up the blend options to fine-tune the blend.

6: Change the spacing to “Specified Steps” and set the number to 700.

7: You should see 700 evenly spaced dots! In order to get them to start off sparsely spaced and then get closer together, we’ll modify the path of the blend. Select the Anchor Point Tool (Shift + C).

8: Use the white arrow to select the first point and drag it until you see the spacing start to change.

9: Create another guide halfway along the path, and then drag the anchor point (with the white arrow selection tool) just past the guide. Repeat the same process with the last circle, but move its anchor point about half an inch to the right.

10: You should now have a gradient spacing of 700 circles becoming more closely spaced as you move from left…

11: …to right.

12: Once you are satisfied with the blend, expand it.

13: ALT+Click and drag the blend to copy it, and place it at the bottom of the rectangle.

14: Select both blends and then use the instructions from Step 4 to create another blend.

15: Use the instructions from Step 5 to fine-tune the new blend.

16: We chose to give our pattern a little bit of randomness, so we ALT+Clicked and dragged the blend down and to the right ever so slightly.

17: With that, we’re ready for the laser cutter! This process is pretty quick to prototype (roughly 15 minutes from start to finish), so explore different gradient patterns and techniques, and try light pipes that are curved or bent (just keep in mind the critical angle).

18: Next, we raster the dot pattern on 1/4″ thick acrylic. Be sure the acrylic surface is clean and scratch-free.

19: Different laser cutters have different settings for varying materials and thicknesses, so keep that in mind when choosing your settings. We used a speed of 80, power of 50 and maximum PPI for the raster. For the vector cuts, we took 4 cuts at speed of 4, power of 95, and maximum PPI.

20: Now take your light guide and examine the edges. Make sure the material is transparent; otherwise you’ll need to do some hand-polishing.

21: The slightly bumpy yet clear finish the laser leaves after a cut works great for our purposes. You’ll want to surround all clear surfaces with white reflective paper or tape, and mask off the quarter inch where the light first enters the pipe.

22: Here are the results with a super-bright LED! You can see that the light starts to drop off a little toward the end of the pipe.

Build Your Own Light Pipe

23: We could come back to tweak the blend pattern (reducing the number of dots at the beginning of the light pipe and increasing it toward the end).

Article is posted by Opticsforhire.com –  An Optical Design Consultant.

Best Practices for Better Imaging System

Whether your application is in machine vision, the life sciences, security, or traffic solutions, understanding the fundamentals of imaging technology significantly eases the development and deployment of sophisticated imaging systems. While advancements in sensor and illumination technologies suggest limitless system capabilities, there are physical limitations in the design and manufacture of these technologies. Optical components are not an exception to such limitations, and optics can often be the limiting factor in a system’s performance. The content provided in this guide is designed to help you specify an imaging system, maximize your system’s performance, and minimize cost.

In this blog you will read a compiled number of best practices for creating sophisticated, cost-effective imaging systems that are applicable for most applications. While the following list is nearly exhaustive and should be considered when designing any imaging system, every application is unique and additional considerations may be required.

Bigger, in many cases, is better. Allow ample room for the imaging system – Understanding a system’s space requirements before building is especially true for high resolution and high magnification requirements. While recent advancements in consumer camera technology have yielded strong results in a small package, they still do not approach the capabilities required for even intermediate-level industrial imaging systems – partially because of their size limitations. Many applications can require complex light geometries, large diameter and long length lenses, and large cameras, in addition to the cabling and power sources required to operate some of the equipment. Avoid having to make sacrifices to system performance just because the system’s space requirements were not considered. It is often advantageous to specify the vision portion of a system first, as it is typically easier to arrange the electronics and mechanics around the vision portion rather than the other way around. It is also important to remember that the illumination scheme is part of the vision system, and the geometry of the object under inspection can often necessitate the use of a large light source such as a diffuse dome.

Don’t believe your eyes – The human eye and brain work together to form an extremely advanced imaging and analysis system that is capable of filling in information that is not necessarily there. Additionally, humans see and process contrast differently than imaging systems. Software analysis should be used to ensure image quality and performance requirements are met. Images that look good to a human viewer may not be usable with an algorithm.

Danger! Don’t get too close – Due to the constraints of physics, attempting to look at fields of view that are too large relative to a lens’s working distance places excessive demands on the design of the optical component and can decrease system performance. It is recommended that a lens be chosen such that the working distance is roughly two to four times as long as the desired field of view is wide, in order to maximize performance while minimizing cost and complexity. Remember Point 1 and consider the imaging system’s space requirement before building the system. This practice also applies to the relationship between sensor size and focal length. It is best to have focal length to sensor diagonal ratios of two to four to maximize performance.

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Light up your life. It really does matter – While it can seem like an art form, selecting the appropriate lighting geometry is highly scientific. In order for a lens and sensor to effectively work together, strong contrast must be produced by properly lighting the object. The characteristics of the object under inspection and the nature of any defects must be understood so that the proper illumination geometry is used. Keep in mind that sometimes these lights can be very large. Learn more about illumination geometries in our Blog on Choosing the Correct Illumination application or) selected for the illumination can have an enormous impact on improving or reducing system performance. For instance, in an application using both high quality optics and a top-of-the-line sensor, switching from broadband to monochromatic illumination, or between specific wavelengths, can improve performance by a significant amount. As with Point 4, the proper choice of wavelength can make the difference between high contrast and no contrast. Depending on whether the wavelength is correctly chosen or not, the color of illumination can determine the success or failure of a system. Learn how proper filtering techniques can have an impact on system performance in our Filtering in Machine Vision application note.

There can be only one; high resolution and large depths of field struggle to coexist – As shown in f/# (Lens Iris/Aperture Setting), maximizing resolution and depth of field requires the same variable, the lens’s f/#, to move in opposite directions. Essentially, it is impossible to have very high resolution over a large depth of field. Physics dictates that this cannot be done and compromises will need to be made or more elaborate solutions, such as using multiple imaging systems, will need to be employed.

There is no universal solution; a single lens that can do everything does not exist – As resolution requirements increase, the ability to decrease aberrations (attributes of optical design that adversely affect performance) becomes increasingly difficult over a wide range of working distances and fields of view. Even without budget constraints, there are limitations. For this reason a wide range of lens solutions for similar applications are required.

Thoroughly understand the object to be inspected – The foundation of imaging is the ability to produce the highest level of contrast possible on the object under inspection, so an understanding of the object’s properties, such as its materials or finishes, is critical to the application’s success. Additionally, it is not enough to just know what parts are considered good or bad. Rather, to guarantee high levels of reliability and repeatability, the range of details that will be inspected and the margins for good and bad must be understood.

Be a control freak – The ability to control the environment into which the imaging system is deployed can significantly affect the reliability and consistency of results. Additionally, it also reduces the likelihood of unintended problems. Whether using filters to increase contrast, baffles to eliminate unwanted light from entering the system, or measurement devices to monitor light sources for spectral stability, controlling the environment will reduce unforeseen difficulties in the future. Some of these techniques are extremely low cost ways to protect and increase the performance of an expensive imaging system.

Be the squeaky wheel – Do not be afraid to ask why something will or will not work. Suppliers should be able to explain why different components in the system are or are not capable of achieving the desired result. The answer will not always be the same; sometimes the issues are laws of physics limitations and sometimes they are deficiencies related to the design or fabrication of the component. Optical manufacturing is a science, and the designers and manufacturers should be capable of explaining why things are happening.

Make a list; understand and define the fundamental parameters of the imaging systems – By narrowing down the specific parameters required for the imaging system, the wide range of available lenses and sensors can be reduced to a manageable selection of components. Fundamental parameters of an imaging systems are a great place to start and are detailed in the next section.

Article is posted by Optics For Hire – lens design Consultant.

 

 

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5 Fundamental Parameters of an Imaging System

The following parameters are the most basic concepts of imaging and are important to understand when studying more advanced topics:

  1. Field of View (FOV): The viewable area of the object under inspection. This is the portion of the object that fills the camera’s sensor.
  2. Working Distance (WD): The distance from the front of the lens to the object under inspection.
  3. Resolution: The minimum feature size of the object that can be distinguished by the imaging system. Learn more in Resolution.
  4. Depth of Field (DOF): The maximum object depth that can be maintained entirely in acceptable focus. DOF is also the amount of object movement (in and out of best focus) allowable while maintaining focus. Learn more in Depth of Field and Depth of Focus.
  5. Sensor Size: The size of a camera sensor’s active area, typically specified in the horizontal dimension. This parameter is important in determining the proper lens magnification required to obtain a desired field of view.
  6. PMAG: The Primary Magnification of the lens is defined as the ratio between the sensor size and the FOV. Although sensor size and field of view are fundamental parameters, it is important to realize that PMAG is not.

                PMAG = Sensor size(mm)/field of view(mm) (1)

Note: Typically, only horizontal values are used.

Diagram of Fixed Focal Lenses

Fig1: Diagram of Fixed Focal Lenses

Article is posted by Optics For Hire – lens manufacturing Consultant.

The Fundamental Aspects of a Light Guide Panel

A light guide panel (LGP) is an acrylic light guide panel made from pure Poly (methyl methacrylate) (PMMA) resin. It has two main sections:

  1. The bottom section where either a dot matrix is printed or a line matrix is scratched.
  2. The edges where the light source is installed.

At any given instance,when light source is directed towards the light guide panel,the light is distributed evenly over the upper surface of the panel. It was mainly designed to optimize the uniformity of light distribution,which makes the backlight slim and as a result,reorients the line or dot light source (e.g. from LED or fluorescent lamp) to the plate light source. There are a number of LGP which have been designed to be used for in-store decorations,lighting equipment etc. In situations where they have been designed to use LED as the main source of energy,they can reduce the power consumption significantly.

The light guide panels have quite a number of specifications which you need to consider when you are planning to purchase them for any application. In this case,the choice of material is never an element to consider since all LGP are manufactured from acrylic material however,they vary in both thickness and weight. For instance,the most common types of LGP have a thickness between 5mm and 8mm with the weight varying between 10kg and 22kg. This does not imply that these are the only sizes which are available in the market.Different companies design light guide panels of different sizes. You need to know the exact surface pattern of LGP you would wish to buy.

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Broadly, there are two most common types of surface patterns which can be classified as:a one sided light emission with a back side dot pattern and a two-sided light emission with both faces with a dot pattern. Of course,there is a lot of scientific research going on in the LGP industry with an attempt to produce cost effective and efficient products. For instance,the introduction of slim LGP which had been designed to be used with LED sign boards did revolutionize this industry. Since then,most products which have so far been launched feature the edge-light system (ELS).

The ELS-LGP are commonly used in a number of indoor and outdoor applications. It is worth noting that these products are thinner and have the capability of illuminating a larger surface area. Moreover,they are cost effective since they have reduced the installation costs and the running costs. In fact,with the advancement of the LGP technology,sign boards which are as large as 1.5m by 2.3m can be illuminated conveniently.

To improve their effectiveness,they are also fitted with a reflective sheet. This basically implies that whenever light is emitted from the source,it travels through the panel in a controlled manner within the interior of the LGP. It is again radiated via the diffusion sheet which is part of the panel’s surface. The major advancements in this industry have been focusing mainly on the following key areas:

  1. The ability to make very thin sign boards using the available light guide panels. This can only be realized by not attaching luminous sources to panel’s edge.
  2. It eliminates the patchy appearance which is a common scenario with fluorescent sign boards by ensuring an even distribution of light.
  3. Introducing eco-friendly products by ensuring that small amount of energy is used in the entire process and reducing the carbon dioxide emissions. In addition to these,the recent development has also championed for the use of mercury free light sources.
  4. It has reduced costs significantly by replacing fluorescent light sources with LEDs which has a longer service life thereby reducing maintenance costs significantly.

Buyers can choose from the three combination of products which are available in the market which include:acrylic LGP which a diffusion sheet;acrylic LGP with both LED modules. It follows that,this technology is really essential in creating billboards,information boards,hanging signs, in-store menu signs etc. The main features which are associated with the light guide panels include:

  • They are energy efficient – The cost of energy has been increasing in the recent past thus,any process which can save significant amount of energy ought to be implemented. The use of LEDs as opposed to fluorescent lamps can reduce energy consumption by between 70% and 80%.This has been one of the main reasons why a number of companies have been adopting this technology.
  • Safety – The LEDs which are used in the LGP operate at about 2°C above the room temperature in most cases. This reduces the risks of fire.This is not the case for the fluorescent lamps which dissipate a lot of heat.
  • Reduced maintenance costs – LEDs have a longer life span that the ordinary fluorescent lamps.This reduces the maintenance costs significantly since the service life of LEDs is ten times more than the fluorescent lamps.

In addition to these,it is also important to note that LGP are transparent since they are constructed using acrylic thus,they can achieve a transmittance of about 92%. It is quite evident that this technology has quite a number of advantages more so when it is being used as a way to advertise products and services. This is due to its efficiency,reliability and cost effectiveness.

To enjoy all the benefits they are associated with,there is only one way to go about the entire process.

You need to get a professional and a reputable company which will take you through the entire design process and the implementation. Get the LGP products from Excelite since you will get a professional guidance about light guide panels. There are quite number of companies which claim to offer such products however,what makes Optics For Hire to standout is their commitment to ensure that all customers get quality products and essential resources they need.

The Five Building Blocks Of An Efficient, High-Brightness LED Driver (Part – II)

Five Building Blocks Of An Efficient LED Driver

HYSTERETIC CONTROLLER

The main function of the hysteretic controller is to regulate current through the LED. A reliable hysteretic controller may use an SR-type flip-flop where the “Set” input is triggered when the current falls below the lower threshold, and the “Reset” input is triggered when the current rises above the upper threshold. By using digital-to-analog converters (DACs) to produce the reference voltages, a hysteretic controller can be made programmable.

With resolution defined by the capability of the DACs, the higher and lower reference values can be controlled to change the position of the ripple current. Reducing the amount of ripple allowed in the channel decreases the ramp times, increasing the switching frequency. Drivers that can work at higher frequencies (ranging from 500 kHz to 2 MHz) can allow for significant reductions in the cost and size of magnetics. In addition, the controller must be able to perform a logical AND of signals from the modulator and trip circuitry.

CURRENT SENSE AMPLIFIER

A high-side sense amplifier allows the hysteretic controller to sense both the rising and falling current ramps of the inductor. Such a CSA needs to differentially sense the voltage and level shift it to the same reference voltage as the hysteretic controller. The CSA operates by creating a current ISENSE in the low-voltage realm that is proportional to VSENSE on the high side. An additional amplifier with adjustable gain can be used to obtain a signal whose voltage matches that obtained from the reference DACs in the hysteretic controller. A high gain setting in the CSA enables the use of low-value sense resistors, minimizing the power losses, and a choice between 20 and 100 should address the requirements of most HBLED designs. Since the CSA is sensing the rising and falling currents, it is important for the sensor’s bandwidth to be greater than the switching frequency. When high bandwidth is not required, choosing a lower one will reduce the noise picked from the supply through the positive pin of the differential amplifier.

GATE DRIVER AND FET

As the gate driver and FET are intrinsically tied to the maximum switching frequency possible and efficiency of the system, they have to be chosen based on a tradeoff between the cost, size, and performance of the design. A FET with lower RDS will reduce conduction losses, and lesser gate capacitance will reduce switching losses. The gate driver must be able to drive the gate capacitance of the FET at the switching frequency desired. If the gate driver isn’t powerful enough, the ramps rate could be too slow, causing the FET to operate in the inefficient linear region. If it’s too powerful, the FET could ring, producing electromagnetic interference (EMI) emissions.

MODULATOR

The modulator’s output provides the dimming signal to the hysteretic controller. A high output from the modulator produces constant current at the LED while a low output relates to zero current. The choice of modulation scheme should allow for a high degree of resolution to harness the potential of LEDs. As the human eye can perceive small gradients at lower intensity levels, an 8-bit modulation scheme will create undesirable and perceptible steps in an extended fade sequence. The higher resolution of a 12- to 16-bit modulator requires a clocking frequency that allows for a smoother gradient.

The modulator frequency must be high enough to allow for a refresh rate that is higher than the persistence of human vision. When using a 16-bit modulation at 700 Hz, the modulator must be clocked at 700 Hz × 65536 cnts ˜ 45 MHz. Today, different modulation schemes are available for driving LEDs. Pulse-width modulation (PWM) involves representing the desired dimming quantity as a ratio of width of the pulse to the period of the pulse. Additional modulation techniques such as precise illumination signal modulation (PrISM) spread the dimming quantity in a pseudorandom fashion throughout the period of the pulse. Such a stochastic signal density modulation scheme spreads the energy throughout the spectrum, reducing quasipeak emissions.

TRIP CIRCUITRY

Various scenarios require the driver element to halt the constant current hysteretic control loop. Operating under sudden input voltage fluctuations and temperature gradients can affect the longevity and performance of the LED engine. Trip circuitry comprising a programmable DAC and comparator can deliver the required logic signal to the hysteretic controller’s logical AND function.

Advances in semiconductor technology are allowing for integration of these components into fast shrinking and inexpensive programmable controllers. The PowerPSoC family of parts includes hysteretic controller channels that can be set up to create various topologies to drive HBLEDs. By coupling integrated drivers with an onboard microprocessor, the cost and form factor of a solution can be reduced with supplementary benefits associated with reduction in EMI emissions.

Optics for Hire is one of the leading companies having highly skilled and knowledgeable Ukrainian optical design consultants. The experienced team at Optics For Hire of designers and engineers are constantly working hard to meet the optics design needs of varied industries. Optical instruments are created for use in optical media, biomedical devices, illumination and imaging systems and metrology tools. The engineers of the Optics for Hire will sit with each client to understand their specific needs and based on the discussion will start with the designing work.

To Read (Part – I) Click Here.

The Five Building Blocks Of An Efficient, High-Brightness LED Driver (Part – I)

Five Building Blocks Of An Efficient LED Driver

As high-brightness LEDs (HBLEDs) penetrate all avenues of the lighting market, various semiconductor manufacturers are offering constant-current drivers. Only by choosing a driver IC capable of meeting the flexibility and control required by today’s applications can the true potential of HBLEDs be unleashed. Theatrical lighting, for example, often requires high dimming resolution while dynamically adjusting the current to account for fluctuating power sources and operating temperature. Since the quality of light output is intrinsically tied to the capability of the LED driver, it is important to choose a system that has the right specifications.

Today’s HBLEDs typically have a nominal current rating of 300 to 700 mA. As the envelope of light output is pushed, devices requiring more than an ampere are appearing in the market. In all LEDs, due to the voltage-current relationship and the binning approach used by manufacturers, a constant-current source is used for accurate control of the light output. Choosing the right constant-current regulator depends on the operating voltage of load and source, the desired efficiency, and the cost and size of the system.

A high-power resistor in series with LEDs would be the simplest form of current regulator. However, since it alone cannot adapt to changing source voltages or the non-linear VI characteristics of an LED, a closed-loop system that changes the resistance based on output current may be used. In either case, the energy not used by the LED is dissipated as heat by the linear regulator, leading to an inefficient system. In most HBLED applications, switching regulators offer better efficiency over a wide range of operating voltages.

HBLED lighting fixtures designed to replace incandescent and fluorescent bulbs must provide better efficiency and quality of light while maintaining low cost. An integrated switching regulator used in lighting applications must require minimal external components and have good current regulation.

DIFFERENT TOPOLOGIES

While switching regulators can have diverse forms, they all operate using the same principle of moving small and controlled quantities of energy from the source to the load. The type of topology that is chosen depends on the type of conversion that is required. A boost topology is used when source voltage is lower than the required load voltage, while a buck allows the source voltage to be greater than the load voltage and is typically used for driving LEDs.

The main control system in any buck regulator is the hysteretic controller. This block regulates the current through the inductor by turning on a switch when it is below the lower threshold and vice versa. A shunt resistor is a convenient method of sensing the current, and by pairing it with a differential current sense amplifier (CSA), a smaller resistance can be used to minimize power losses. The analog circuitry of the controller uses the feedback from the CSA. These blocks can be arranged in various combinations. Different LED colors differentiate the topologies.

In all three topologies, current flows through the inductor when the corresponding switch (field-effect transistor, or FET) is turned on. When the current rises above a predetermined limit, the hysteretic controller on each topology turns off the FET. As the current in the inductor persists, it conducts through the flyback diode until it falls below the lower threshold and the FET is turned on again. A system capable of faster switching will require smaller inductors to store magnetic flux between alternate cycles.

The topology with the red LED is configured with a low-side sense resistor located on the source pin of an N-FET. An inherent problem with this implementation is that current through the inductor can only be sensed when the switch is on. Once the current reaches the peak threshold and the switch is turned off, the hysteretic controller must use a timing circuit to turn the switch back on.

If during the off cycle the falling current does not reach the lower threshold or overshoots it, the off-time must be adjusted until the loop is stable at the required current ripple. As this technique has true hysteresis on only one side of the loop, it won’t be able to quickly adjust to fast transients of source and load conditions.

A hysteretic control system that can sense both falling and rising edges requires the feedback loop to remain in the current path regardless of the state of the switch. The topology used by the blue LED shows the sense element in the path of the inductor current in the charging as well as discharging phase. To achieve this, a high-side switch or P-FET is used. Because the RDS (resistance offered by the FET to current) is higher in P-FETs than in N-FETs, there is a loss in efficiency. Additionally, the high-side driver and the P-FET itself are generally costlier than a low-side driver and N-FET rated for the same switching capability. Finally, in the topology used by the green LED, the FET and sense resistor swap positions. This permits the use of an N-FET to increase efficiencies while the location of the sensing element allows inductor current to be sensed throughout the operation of the hysteretic controller.

Working as a system, the LED driver channel depends on five elements to create a topology that is efficient and robust while meeting the demands of HBLED applications: the hysteretic controller, the current sense amplifier, the gate driver and FET, the modulator, and the trip circuitry. The same blocks may be used for other topologies such as boost, buck-boost, and single-ended primary inductor convertor (SEPIC). To Read (Part – II) Click Here.

Article is posted by Optics For Hire – Optical Design Consultants