New Chromaticity Diagram



Date: 02 JULY 2009

[1.3.1] Some Simple Reality Checks

Before going too far, exploring some simple arm chair experiments in human vision is useful. This will better enable the reader to evaluate some statements of folklore, found in both the popular and scientific literature, that have risen to the level of an axiom.

[] Adaptation

--There is frequent reference to the synchronous adaptation of both eyes in response to a change in light level to either eye. While in a dimly lit room, less than one-half moon outside, close one eye and turn a light on for a few seconds to a minute. Turn off the light. Now observe how much detail you can see with the open eye. Close the first eye and open the other eye. Observe how much detail is visible. Alternate eyes a few times. The level of adaptation is clearly quite different in the two eyes; although looking in a mirror will confirm that the irises of both eyes have contracted to the level established by the eye open to the higher light level. Conclusion, the irises of both eyes normally operate in synchronism, however, the major adaptation process in vision operates individually in each eye and is independent of the iris.

[] Peripheral Color Vision

--Look straight ahead while bringing a reasonably large (1/2 inch square) object of red or blue from behind you into your peripheral field of view. Note the angle when you first see the color of the object. This experiment should clearly show you have color vision at angles greater than 60 degrees from the optical axis, the area limited to "rod" vision in both the scientific and popular literature. The status of "rods" will be developed later.

[] Foveal vs peripheral night vision

--On a clear night, take your star map and go outside to look at the stars. It is best to be thoroughly dark adapted (at least 20 minutes to be fair). However, the point can be made with less dark adaptation. Our object is to see how much more sensitive our peripheral vision is to our foveal vision. The most sensitive part of our field of view is about five degrees temporally (toward our ears in the horizontal plane) from our point of fixation. This is the location of the optical axis of the eye. The photoreceptors located there are about 50% larger in diameter than in the fovea. Our object is to see what is the dimmest known star we can see using our most sensitive area and using our fovea. Don't use a red star in this initial experiment. Most people will find the difference in stellar magnitude between these two stars is less than one. One stellar magnitude is a factor of 2.5:1. This is a difficult experiment because of the small difference in sensitivity actually involved. Try using the stars of Ursa Major (the big dipper). Can you see all four stars forming the cup of the dipper with both your foveal and your most sensitive vision.

Clearly, our night foveal vision is not limited by "cones" in the fovea, with a sensitivity 1000 times less than "rods," as frequently stated in the literature. It is limited by the smaller cross-sectional area of the photoreceptors in the fovea and the poorer performance of the elliptical lenses of the eye five degrees from the optical axis when the iris is fully opened (related to the Stiles-Crawford Effect). The first cause introduces a factor of about 2:1 and the second cause accounts for a factor of less than 1.5:1.

[] Night Color Vision

--Step outside on any clear night and look up at the stars. Do you see any colored planets or stars? Mars, "the red planet", should be easy to see somewhere along the ecliptic if it is in the sky. There are many colored stars, and many of these have names given to them in ancient times. According to the Field Book of the Skies, "There is a wide variety of tints easily seen with the unaided eye." These usually range from green to yellow to red. Can you recognize the color of a star while looking directly at it? Do you have color vision under low light conditions in your fovea?

[] Eye as a Camera

--While looking straight ahead and without moving your head, note closely the field of view you perceive. Without moving your head or blinking, move your line of sight 5-10 degrees to either side and back. Try up and down by 5-10 degrees. Did the overall scene that you perceive move? Would you expect this result if you took a picture with a camera and then took a second picture with the camera turned 5-10 degrees? The process of detecting and perceiving information by the eye will be addressed later.

[] Eye as a Change Detector

--During your next eye examination, while sitting at the Visual Field Analyzer --a device that presents a uniform white field to the eye-- repeat a simple experiment fully explored by Yarbus in the 1950's. Before the technician inserts a stimulus into this uniform white field, note that in the absence of any blinking or head movement, you begin to observe a darkening of your field of view after 1-3 seconds. If carefully done, you will become completely blind during this experiment. This is not the normal result obtained with a camera. It is the normal response expected of a sensor that is a "change detector."

[] Bright Adaptation as opposed to Dark Adaptation

--After attending an afternoon movie and becoming fully dark adapted, walk quickly out of the lobby of a small theater (or out the emergency exit) into the bright sunlight. Neglecting the pain for the moment, how long does it take before you can see effectively? It probably takes a few seconds, the period required for "Bright Adaptation." Both "Bright" and "Dark" adaptation need to be considered, explained and quantified.

[] Noise Limited performance--

[[or photon noise limited performance ]]

These simple experiments will be addressed later to help the reader put into perspective and better understand some of the results presented.

The numbers in [brackets] refer to the paragraph numbers in the main text.