Luminance Contrast Measurement of Displays
by Hemispherical Illumination

Indirect incident light falling on displays, as under day light conditions, reduces display contrast by washing out blackness and color. For product research and quality control the Luminance Contrast and Luminance Color Contrast of displays is typically measured. The highest possible contrast measured is dark room contrast. Ambient contrast specifies actual contrast conditions. For ambient contrast measurements the display under test must be indirectly illuminated. A simple way to simulate indirect lighting is through hemispherical illumination using an integrating sphere light source. This technical note describes different ways to use integrating spheres for hemispherical illumination.

Luminance describes the photometric brightness of extended surface light sources (reflected, transmitted or emitted light). Luminance contrast describes the difference in luminance between adjacent surfaces. For displays luminance contrast describes the difference between the maximum and minimum detectable luminance value. Luminance color contrast describes the uniformity of color at different luminance levels. The measurement of the luminance contrast and luminance color contrast can be done under both dark room and in ambient light conditions. For dark room condition testing the luminance detector is typically placed on the surface of the device under test (contact measurement). For ambient light condition testing the device under test must be hemispherically illuminated to simulate indirect ambient lighting. The luminance detector is placed at some distance from the device under test.

Integrating Spheres for Hemispherical Illumination:
The reference point for hemispherical illumination is at center of the integrating sphere. So the ratio of the sphere diameter to the surface dimension of the device under test (DUT) should not be too small to avoid any non-uniformity in hemispherical illumination of the sample. The light source can be placed inside the sphere (IS) or outside the sphere (OS). For IS position, using bulb type lamps, all luminous flux is available for DUT illumination. The useable flux of external lamps in OS position is less but DUT illumination level is adjustable by controlling the flux going into the sphere using variable apertures. This has the added advantage of controlling light levels without effecting the lamp spectra. The luminance meter ’looks’ at the DUT by mounting  the detector to a port opposite to the DUT surface. This port acts like a light trap which avoid perpendicular light incident on the DUT. In some applications the luminance meter port is orientated at an 8° angle to the sphere center. An other port at –8° position supports the ambient contrast measurement with or without gloss component. Here the –8° port can either be closed with a port plug or a light trap. For the measurement of spatial luminance distribution ports with a different orientation to the DUT can be used. And for measurements requiring a higher viewing angle resolution a goniometric drive can be placed inside the sphere for two axis positioning of the DUT.

Luminance and Luminance Color Meter or Observer:
For luminance color contrast Gigahertz-Optik’s X4 light analyzer with LDM-9812 luminance detector is used. The Bi-Technology concept of this measurement device provides accurate measurements for all types of display backlighting sources. For luminance contrast measurements without color Gigahertz-Optik’s luminance spot meter models LM-9901, LDM-9810 or SRT are available. Beside using light meter integrating sphere systems for hemispherical test device, the integrating sphere source (ISS) can be used for illumination evaluation by an observer as well.        

Integrating Sphere Source (ISS) Design Criteria:
Five different principal design configurations are possible:
1. Hemisphere Integrating Sphere Source
2. Integrating Sphere with Sample Port
3. Integrating Sphere with Hinge Frame
4. Integrating Sphere with Goniometric Sample Holder
5. Integrating Sphere Source for Observer    

1. Hemisphere ISS:
This half-sphere set-up with one hemisphere is useful in applications where the device under test cannot be placed in the center of a full integrating sphere, or to increase efficiency/speed of sample handling or space problems in automated production control environments. One constraint to this approach is because the bottom plate and the baffle falls within the hemispherical view of the DUT. This effects the uniform light distribution within the hemisphere and therefore the hemispherical illumination of the DUT will be less than in a regular integrating sphere.                

2. Integrating Sphere ISS with Sample Port
For research in the lab and non-production applications where the device is used infrequently the use of integrating spheres with an application port and sample holder are a reasonable alternative to integrating spheres with an internal sample holder requiring a sphere that opens and closes.

3. Integrating Sphere with Hinge Frame
An integrating sphere that opens and closes offers more freedom for handling the device under test and ease of mounting it in the center of the sphere. The hinge frame of this type of sphere allows one hemisphere to open/close while maintaining stability. This type of sphere also supports different size and shape samples that might not fit through the application port of a closed sphere.

4. Integrating Sphere with Goniometric Sample Holder
If the luminance and luminance color contrast at different viewing angles needs to be measured under constant hemispherical sample illumination an unique tool to accomplish this is integrating sphere with integrated goniometric drive of the sample holder. In this set-up the surface of the test sample can be orientated at Phi and Theta angles to the luminance meter enabling measurement of the hemispherical luminance distribution of displays. The sphere configuration with measurement and auxiliary ports in +/-8° orientation to the sample enables the measurement of the luminance contrast with and without gloss by alternating use of a light plug and light trap at the auxiliary port.              

5. Integrating Sphere Source for Observer
In some applications the luminance and luminance color contrast measurement data must be correlated with physiological impression. This is done using a statistically selected group of observers and analyzing their visual impression of the test samples under different lighting conditions. The integrating sphere light source configuration for this type of testing is almost the same as that with the goniometric sample holder described earlier. One difference is that the light source color temperature must correlate to that of daylight which is specified in CIE standards as D65 standard illuminant. Also the illumination of the test samples must be controllable from low illuminance up to 100000lx to simulate different contrast situations. The D65 illuminant and wide dynamic range light levels simulate real daylight condition for the observer. In applications where light meters are part of the system different color temperature corrections and contrast measurements at lower intensities can be done.        

Picture 1: Principal set-up and functional criteria

Picture 2: Hemisphere light source

Picture 3: Principal set-up of a hemisphere source

Picture 4: 500mm diameter integrating sphere with 127 mm application port for sample insertion

Picture 5: 500mm Integrating sphere with hinge frame. One hemisphere opens. 127 mm application port for sample insertion