TV Contrast Ratio Explained - Part 2
The Eye Sensitivity to Contrast and Image Brightness
Is the eye capable of perceiving the latest high CR ratings?
TV makers are doing their best to come up with improved dynamic contrast ratings for their HDTVs. This in itself is something to be commended, but as further expressed under part 1 of this article, an improved contrast ratio rating does not necessarily imply a better TV picture.
In addition, there is another very important issue TV makers would not tell you when they try to impress with their TV contrast ratings, and this is to what extent your eyes will be able to perceive the latest mega contrasts.
You see, the human eye is an issue on its own - with its own strengths and limitations.
In this second part, we discuss the human eye response to variations in image brightness and how this impacts our ability to detect image contrast in a TV picture.
The eye is not an absolute sensor; what the eye sees are brightness differences. Unlike electronic sensors in digital imaging systems, which are all absolute in their response, the eye has a dynamic response to contrast. This is also referred to as dynamic range of the eye.
This alters the whole equation in that what is really important is not some unrealistically high absolute ratio rating, but rather, how the end image is perceived by the eye.
Why is it so?
The eye is an extremely complex sensor with far greater response range than any film or digital imaging device. It is able to function both in bright sunlight as well as in very faint light during the night - that's a range of over 10 million to one! In photographic terms, that's about 23 1/2 stops.
At the same time, it is not possible for the eye to see the faint light from a star in the sky during the day. From a photographic perspective, this is like saying that a camera can operate over an extended range, but then the full operating range is achieved by adjusting the film ISO speed, the camera aperture setting, and even the exposure time.
It is the same with the eye; at any given instant, the eye can possibly see over a range of 400 to 800:1 in contrast detection. Some may even see up to 1,000:1 and possibly a little beyond but the eye would not detect any difference above approximately 1,000:1. For anything above that, the whites would be already too bright and the blacks would be too dark for the eye to distinguish any further detail.
The wider dynamic range of the eye is possible as once the eye moves (saccades), it re-adjusts its exposure both chemically and by adjusting the iris. Over time, it is possible for the eye to resolve a contrast ratio range (sort of the dynamic contrast range in HDTVs) of between 1,000,000 and 10,000,000:1.
However, the eyes' ability to detect contrast depends on the scene brightness, with the contrast sensitivity of the eye decreasing to about 8% of its maximum at low light levels; the eye sensitivity also decreases with lower contrast subjects.
To complicate matters, the eye Contrast Sensitivity Function (CSF) is not linear; in as much as it decreases with a decrease in brightness, there is a brightness level above which the eye contrast sensitivity falls once again. It is estimated that the sensitivity of the eye is some 600 times less in bright sunlight.
At low light levels, the eye is able to integrate image information over a period of some 15 seconds to be able to see dimly lit subjects (it is like when you increase the camera exposure time to record subject information during low light level conditions).
However, moving from one brightness range to another would take time for the eye to adapt. A 'dark-adapted' eye would be able to see a faint star in the sky, but this dark adoption process by the eye takes 30 minutes or so to complete - provided you haven't been exposed to bright sunlight during the day, otherwise, it may take even up to one hour for full adaptation to take place. Looking at a bright subject by a dark adapted eye would simply kill your night vision
This implies: The first 30 minutes or so during a movie are not the best in terms of contrast sensitivity. What's more, looking at a bright movie scene would not help when the next dark scene comes around!
And as if this is not enough, there are other factors as well that affect our contrast perception. For example, the eye would perceive a higher contrast when looking at static images than it would if it were to look at moving pictures even if these were to have similar contrast ratio to that of the static scene.
There is also the issue of the eye contrast sensitivity with spatial frequency. Technically speaking, spatial frequency is the number of cycles subtended at the eye per degree of vision.
It can be thought of gaps between adjacent image parts that vary in contrast. There is a point above which increasing the spatial frequency i.e. decreasing the gap between adjacent image parts, will reduce the eyes sensitivity to contrast even though the contrast level of the image remains the same.
And what about the loss of contrast sensitivity with age? The amount of light reaching the eye photoreceptors diminishes as we grow older; this in turn affects the eyes' response to brightness and contrast perception.
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So what's the whole discussion here?
Do not get carried away with big contrast numbers. The eye dynamic response is the limiting factor; at any one point in time, you would not be able to perceive a contrast ratio of more than at most 1,000:1 - irrespective of display technology.
Equally important is that the presence of even a minimal amount of ambient light would make a home theater projector or a direct-view display device with a high contrast ratio rating behaves the same as one with a much lower rating. Direct-view systems are less affected in this respect though their blacks would still suffer under a bright light environment.
Opt for a high contrast projector or display device only as long as you can view the image in a light controlled room. Despite what might seem to be a huge difference in absolute terms between say a 1,000:1 and a 1,000,000:1 contrast ratio, you will have to view the image in a completely dark room to perceive the resultant minimal difference in picture performance. Paying extra just to enjoy a higher contrast ratio is just waste of money.
Conclusion: Numbers are there to impress and help the marketing people sell the product. But quoted peak contrast ratings do not really give you any information on the video projector or display device ability to render images with lifelike gray scales and colors. Rather, the only information that contrast ratio can deliver is how much brighter the 'whites' can be than the 'blacks'!
References used in this article:
Resolution and other Details of the Human Eye
The Fallacy of Contrast:
Projector Contrast Ratio:
Eye Contrast Sensitivity and Spatial Resolution
Contrast Ratio - What does it really mean?