Ambient Light and Contrast: Calculate the Added Light

Describe what reaches the viewer

Light from the image and light reflected by the screen both reach the viewer. Consequently, a dark-room black level cannot describe a display in an illuminated installation. Record the lighting, screen finish and viewer geometry alongside any contrast value. A bright reflection in the viewing direction may dominate an otherwise dark image.

For an opaque display, use C = (Lw + Lrw) / (Lb + Lrb), where Lw and Lb are dark-room white and black luminances and Lrw and Lrb are the added reflected luminances in those respective states. All terms use cd/m². Reflection can differ between white and black states. The equal-reflection case sets Lrw = Lrb = Lr. This additive model assumes stable operating conditions and excludes background transmission. NISTIR 6738, equation 4 (printed p. 2) uses separate white-state and black-state diffuse reflectances.

Relate lux and reflected luminance carefully

For an ideal diffuse Lambertian surface, Lr = ρE/π, where E is incident illuminance in lux and ρ is diffuse reflectance as a fraction. This equation supplies a screening estimate under its assumptions. A glossy display assembly has directional reflections; substituting a general catalogue reflectance can misrepresent a particular viewing direction.

Measure the powered black state; a powered-off surface may behave differently. Record the illumination spectrum and measurement geometry. A practical comparison should use the complete assembly, including protective glazing. Evaluate actual light-source positions and viewing locations. Keep any direct reflection of a luminaire visible in the test rather than averaging it away in a single overall number.

Turn the model into a site check

1. Identify the normal viewing zone and the least favourable required view.
2. Record daytime, artificial-light and night operating conditions relevant to the site.
3. Measure white and black under one documented mode and test geometry.
4. Change one controllable factor: light-source position, shading, operating preset or approved optical construction.
5. Repeat the measurements and perform the actual reading task.

Compare both image visibility and comfort. Increasing output may create an excessive night-time luminance or additional thermal load. If automatic dimming is used, test sensor placement, minimum/maximum settings and response to transitions. Keep the sensor clear of light dominated by the screen's own output.

For transparent OLED and transparent LED, add the transmitted scene explicitly and test representative backgrounds. The next chapter explains that separate optical boundary.