Figure 3 – Relative brightness perception
The human eye is capable of seeing a total brightness range of 1012 or more, but not simultaneously. For typical light levels the eye has an instantaneous dynamic range (IDR) of ~104 (10,000:1). The IDR may be viewed as a window of dynamic range that can slide up and down the total possible range as the eye adapts to different brightness levels. The center of the IDR is set by the average brightness within our field of view at a particular time.
As the actual brightness increases, though, the eye becomes less and less sensitive to changes in brightness. Said another way, it takes increasingly larger steps of actual brightness in order to realize equal steps of perceived brightness. Due to this compressive behavior the eye perceives just 360 nits as being half-way (50 percent of IDR) to the maximum of 2,000 nits.
Overcoming Ambient Light
So what gets in the way of displaying high contrast images? “The biggest enemy in outdoor or any high-ambient light applications is the reflection of ambient light from the front of a display,” Presley said. “These ambient reflections will effectively limit how black a display can go, akin to raising the noise floor of the display,” he said. “Also, reflected light will mix with the light being emitted by the display, so at the same time the color gamut will be compressed.”
Reflections are often categorized as being specular or diffuse. Specular reflection, also called glare, represents mirror-like reflection. Specular reflection can be an issue for both daytime and nighttime operation, as there can be many relatively strong sources of light at night as well. On the other hand, diffuse reflection represents scatter-like reflection, such as from a piece of rough paper. Diffuse reflection causes an incident ray of light to break into many smaller rays over a full hemisphere of reflected angles. Therefore, objects and light sources become “scattered” and unrecognizable upon reflection.
In reality, practically nothing exhibits purely specular or purely diffuse reflection. Since specular reflection is mirror-like then its effects are viewing angle dependent, whereas diffuse reflection has very low angular dependence. The low contrast image shown in Figure 1 is indicative of a display having a high diffuse reflection because the image is uniformly degraded and there are no obvious mirror-like reflections from any external objects or ambient light sources.
How much contrast ratio is enough? Many commercial displays claim to exceed 1,000,000:1, but this is always measured in a dark room with no ambient light. In truth, relatively modest contrast ratios can be sufficient for maintaining attractive content. Figure 4 below illustrates various contrast ratios via simple grey-scale blocks. Within every block the lighter colored square is the same shade; only the outer ring of each block is being varied. The difference between contrast ratios of 20:1 and 30:1 is scarcely evident, so 20:1 would appear to be a sufficient goal.