Every display that leaves a factory has been measured. Panel makers, module assemblers and device brands check luminance, color and uniformity across the whole screen, and many now correct each panel pixel by pixel before it ships. The instrument behind most of that work is an imaging colorimeter or imaging photometer: a lens, a set of filters and a scientific camera that turns a picture of the display into calibrated numbers.
This post looks at what that camera has to do and the questions instrument builders should settle before choosing one. We use Finger Lakes Instrumentation (FLI) cooled CMOS cameras as the worked example. For a broader look at inspection lines, see our post on cooled cameras for display, solar and optics inspection.
What an imaging colorimeter measures
A spot meter measures one small area of a screen. An imaging colorimeter measures millions of points at once, so it can map luminance and color across the full panel. From those maps come the numbers the line cares about: uniformity, mura (blotchy or banded non-uniformity), white point, color shift and individual pixel defects.
The camera is the measuring element, not just an image source. Its job is to give the same signal for the same light, every time, across the whole sensor. That makes noise, stability and repeatability more important than frame rate or pretty images.
Filters and the filter wheel
Color is defined by the CIE colour-matching functions, published in CIE 15, Colorimetry. A filter-based imaging colorimeter puts filters in front of the sensor so that the camera's response in each channel approximates those functions. The closer the match, often called the Luther condition, the smaller the error when the instrument measures displays with different spectra, such as LCD, OLED and microLED panels.
Most designs place three or four filters in a motorized wheel and capture one frame per filter. That puts two demands on the wheel: repeatable positioning, so each filter returns to the same place, and enough positions for color filters, a photopic filter, neutral density filters for bright screens and a blocked position for darks. FLI filter wheels use unmounted filters, stepper motors and Hall-effect homing, and come in round and square filter formats. Filter thickness shifts focus, so plan the optical path with our post on back focus, flanges and filter wheels. If cycle time matters, our post on fast filter switching shows where the time goes.
Sampling the display
To measure individual display pixels, the camera needs several sensor pixels across each display pixel in each direction. Too few, and the image shows moiré, where the display grid and the sensor grid beat against each other. Enough sampling, and each display pixel can be located and measured on its own.
That is why high-resolution sensors suit this work. The FLI AR411 uses a 14192 × 10640 sensor with 3.76 µm pixels. Imaging a 3840-pixel-wide display across the full sensor width gives about 3.7 camera pixels per display pixel horizontally, or about 3.5 with a small margin around the panel. The AR455, at 9568 × 6380, covers smaller or lower-resolution panels, or a region of a larger one. Larger panels and higher pixel counts can be measured in tiles. Our post on pixel size and sampling covers the matching math.
Low gray levels and long exposures
Mura is hardest to see, and often matters most, at low gray levels. A dark screen sends very little light, so exposures get long, and at every filter the signal must stay well above the camera's noise. Correcting a panel pixel by pixel, often called demura, raises the bar further. The correction is only as good as the per-pixel measurement, and it is usually captured at several gray levels.
This is where a cooled camera earns its place. Cooling keeps dark signal low and steady through long exposures, and back-illuminated sensors collect more of the light that does arrive. The arithmetic is in our post on long exposures and dark current.
Dark frames and flat fields
Accurate maps need two corrections. Dark frames remove the camera's own signal at each exposure time and temperature. Flat fields remove lens vignetting and pixel-to-pixel response differences, so a uniform source reads as uniform.
FLI Aurora cameras are not offered with a mechanical shutter. Darks are taken through a blocked filter-wheel position instead, such as the one on the CFW8-8RND, which carries seven 50 mm round filters plus a blocked position. In a colorimeter that wheel is already in the light path, so the dark position costs nothing extra.
Calibration and repeatability
An imaging colorimeter is calibrated against a reference instrument, usually a spectroradiometer, on known sources. The calibration only holds if the camera behaves the same tomorrow as it did on calibration day. That means a stable sensor temperature, a linear response across the range you use, and consistent gain.
Measure those properties when cameras arrive, not after a calibration drifts. FLI provides a production test summary for each camera by serial number, and data-sheet values are typical. Our post on acceptance testing for gain, noise and linearity lays out the tests. For measurement practice on the display side, the ICDM's Information Display Measurements Standard (IDMS) is a widely used reference.
Integrating the camera into a metrology instrument
Colorimeters are often built into compact heads that mount on a test stand or robot arm, sometimes inside a dark enclosure that also holds the panel driver. FLI Revelation cameras use the same Sony IMX sensors as Aurora in a thin, fin-less body that is shorter front to back and can be liquid-cooled. That suits tight instrument geometries and enclosures where fan air is unwelcome. Every Revelation camera includes our software development kit (SDK), so the camera, filter wheel and display driver can run under your own control software. Cooling choices for enclosed instruments are covered in our post on air versus liquid cooling.
Questions to settle before you choose
- What display sizes and pixel counts must you measure, and at what working distance?
- How many sensor pixels per display pixel do you need, and will you measure in tiles?
- Which filters, and how many wheel positions, including neutral density and a dark position?
- What is the lowest gray level you must measure, and what exposure does that require?
- What measurement time per panel can the line accept?
- How will you calibrate, and how often?
- How much space, heat and airflow does the instrument head allow?
Talk to our sales team
Send our sales team your display formats, sampling target, filter set and cycle time, and we will suggest the standard camera and filter wheel configuration that fits. If an instrument needs a modest change to a standard camera, we are glad to discuss it. Request a quote or email sales@flicamera.com.
FLI cameras are Made in USA, and our sensor chambers are assembled in our 1,000+ sq ft ISO Class 7 clean room. See how on our in-house manufacturing page.
