- Contrast Ratio
- Static and Dynamic Range
- The Dynamic Range of the Human Eye
- Contrast Sensitivity
By painting colors and lines and forms seen in quickened mood I was seeking to make this mood vibrate as a phonograph does – Edvard Munch
Each point in space has its own luminance value. Light reflecting off a tea cup has one luminance value, light reflecting off the tea has another and light reflecting off the spoon hass yet another.
The difference between luminance levels is contrast. We have already discussed this earlier. The ratio between the highest luminance value to the lowest luminance value of an image or scene is the contrast ratio.
When there’s no difference, there’s no contrast (as shown in the first image). When the difference is as stark as having both white and black, an image is said to have maximum contrast.
We have already seen that the total lux levels the eye can accommodate is about 30 stops of light – or 1,000,000,000:1 from white to black. This ratio is called the Static Contrast Range of the human eye.
However, at any given time, the eye can only see a range of 1000:1, or about 10 stops of light. This range is called the Dynamic Contrast Range or simply Dynamic Range – since its value changes over time.
One example of the dynamic nature of the eye’s contrast range is what happens when somebody steps into a dark in a movie theater while a movie is in progress. Initially, the person can see what’s being projected, but the rest of the hall appears pitch black. After a few minutes, the projected screen is still visible, but at the same time parts of the hall is visible as well. For people with good low light vision, most of the hall will be visible, even while facing the screen directly.
Any value that falls on the ‘1’ end of a 1000:1 scene will appear black to the eye, even if it isn’t a black body. Any value that falls on the ‘1000’ end will appear white, or at least very near white. We have already seen that cones are responsible for vision during bright outdoor light. Rods, on the other hand, can take in about 1,000,000:1 or about 20 stops of light.
This means that the human eye has a greater dynamic range in the shadows than in bright light – but only when no bright light is directly hitting our retina – the dynamic range of the eye in the shadows is about 20 stops. Which means the dynamic range of the eye in bright outdoor sunlight is only about 10 stops.
Correction: The middle value in the above image is 1 cd/m2 and not 0 cd/m2 as shown.
Photopic vision is the vision of the eye under well-lit conditions – in humans it gives the best color, image resolution and temporal resolution. Photopic vision is purely cone-based.
Mesopic vision is a combination of photopic vision and scotopic vision in low but not quite dark lighting situations – like in traffic. It is interesting because this vision requires both cones and rods.
Scotopic vision is the vision of the eye under low light conditions. It is almost monochromatic in nature is is purely rod-based.
The eye is so good at adjusting between bright and not-so-bright situations, like when you are looking outside the window and turn back in to face somebody in the other end of the room, that even though the eye can only see 10 stops of dynamic range at any given time, our perception is that we’re seeing more than that.
Does the dynamic range only depend on external light levels? No. It also depends on the frequency of detail in the scene.
The eye is most sensitive to contrast differences at a frequency of 4 cycles per degree (about 7.5 arc minute). This sensitivity, called the Contrast Sensitivity of the eye, is higher than what the eye is capable of at 60 cycles per degree (about 0.4 arc minute).
All in all, a good estimate for the dynamic range of the eye is about 20 stops. Why? Because then we’re within the bounds of either the mesopic phase or the photopic phase. Any images that have more than 20 stops of dynamic range look unnatural to the eye.
- The difference between the brightest luminance in a scene/image to the darkest is called contrast.
- The ratio of the brightest luminance to the darkest luminance is called contrast ratio or dynamic range.
- The dynamic range of the human eye is about 20 stops, or 1,000,000:1.
- The contrast sensitivity of the human eye is greatest when the frequency of detail in a scene is at about 4 cycles per degree.
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