Measuring the Colour Uniformity of LED Arrays

The problem of LED colour consistency (the lack thereof) is well known, which is why those who manufacture LED-based products will usually purchase LEDs based upon agreed colour tolerance specifications, that is from defined "bins" (or sub-bins). Bins define small, 2D zones within the CIE colour space diagram within which the chromaticity coordinates of the batch of LEDs will fall. This way, you can be sure that all of the LEDs from that batch should look a similar colour (although rarely will they look identical, such is the high level of sensitivity to colour difference that the human vision system possesses). So specifying the LED colour bin solves the problem of variable colour from LED-to-LED or from batch to batch? Not quite. What if the LED vendor ships you LEDs from the wrong bin? What if you populate your LED board with LEDs from the wrong batch? What if two different vendors supply you with LEDs that are meant to be the same colour but aren't? What if the LED vendor's quality control processes mean that the colour variation exceeds the specified tolerance of the bin? These and other issues can mean that your LED-based product is aesthetically and/or functionally prejudiced by a lack of colour consistency, both LED-to-LED on the same board and from board-to-board.

The issue of determining the variation in colour LED-to-LED on an LED board (typically one that forms the basis of a luminaire) is one that is frequently raised by customers of the Photometric Testing laboratory. Measuring the colour and brightness (luminance or luminous intensity) of each LED on a circuit board that may contain hundreds of LEDs is possible but can be very time consuming. Fortunately, here at Photometric Testing, we use several imaging colorimeters, supplied by our sister company Pro-Lite Technology. An imaging colorimeter makes the job of measuring the colour of each LED in an array impressively simple, almost as simple as taking a picture. Which is what the imaging colorimeter does, as it happens!

Think of the imaging colorimeter as like a high resolution digital still camera, except that the pictures that it takes are photometrically and colorimetrically calibrated and possessed of a photometric sensitivity and dynamic range that exceeds even that of the human eye. With up to 16 million pixels in the camera's CCD sensor and with up to 16 bits (65,000:1) dynamic range, an imaging colorimeter can accurately record the colour and brightness of literally thousands of LEDs in an instant, thus making it possible to automatically analyse an LED array in real time in a production environment. The picture below shows a false colout luminance map image of an LED array with software-applied virtual "areas of interest" detectors applied to report the photometric brightness and correlated colour temperature of three of the emitters.

LED Array

Templates (pre-determined grids) of analysis points can be stored in software to simplify the analysis of different designs of LED arrays, so all the operator has to do is align the imaging photometer to the device under test, choose the appropriate point of interest set, record an image and review the analysis report to determine whether the LED array in question meets defined pass / fail criteria. You can set upper and lower limits both in terms of brightness and CCT that the individual LEDs or the board as a whole have to meet. You will also know whether any LEDs on the board are unlit. For automated production line testing, a software API (programming interface) allows the imaging colorimeter to be integrated into an on-line test station. Not only does the imaging colorimeter guarantee the colour consistency of the LED array, it also replaces human visual inspection, which can be both subjective abd unreliable and of course extremely unpleasant to perform, given the glare from high brightness LEDs.

Photometric Testing is pleased to offer contract testing of LED boards. Please feel free to call us on
+44 (0) 1242 701300 for a quotation, or use our on-line enquiry form.