The Human Eye

Let us examine the human eye as a prelude to setting up a color-specification method. The optical parts of the human eye are  

The cornea is the outer entrance to the eye. The iris is controlled by muscles so as to adjust the pupil of the eye according to ambient light intensity. The lens focuses light upon the retina and its shape can be somewhat altered by musculature control. The retina consists basically of a mosaic of light-sensitive cells, each operating on a photo-chemical principle. There are two (2) types present the rods and the cones . The basic difference between the two is that cones are sensitive to color whereas rods have little color response and are operative primarily when 

Photopic vision peaks at 5500 A whereas scotopic vision peaks at 5200 A. The following, given as 6.7.7. on the next page, shows both the photopic and scotopic response curves for the human eye, as determined from a number of observers. In this case, the relative response of the observers are summed into a response called THE STANDARD OBSERVER and is normalized for easier usage. You will note that these eye-response curves are the result of an average of many human eye response curves. 

let us examine the effects of colors as perceived by the human eye. b. The Nature of Chroma 

The visible spectnun extends from about 4000 A to 7000 A. We find that the eye acts as an integrating instrument. Thus, two colors may appear equal to the eye even though one is monochromatic light and the other has a band of wavelengths. This is shown in 6.7.8., given on the next page. In this case, we may see the same color, but the photon energies are much different. 

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The smallest difference of the optical density which the human eye can discern is approximately AD = 0.01 at density 2-2.5 provided the two areas with this density difference are of homogeneous density ( no noise) and of sufficient size, respectively. The limiting values of the granularity of the classes Cl to C6 of EN 584-1 vary between 0.018 to 0.039, ... 

This relation is only valid for small Ad and small lateral extensions. The influence of the inherent unsharpness is not taken into consideration and besides this the ability of the human eye to integrate over an area for noise reduction is not considered, which would have positive effects on the perception oflarge or longish flaws (or wires). 

Plenary 8. J Grave et al, e-mail address J.Greve tn.utwente.nl (RS). Confocal direct unaging Raman microscope (CDIRM) for probing of the human eye lens. High spatial resolution of the distribution of water and cholesterol in lenses. 

The first detector for optical spectroscopy was the human eye, which, of course, is limited both by its accuracy and its limited sensitivity to electromagnetic radiation. Modern detectors use a sensitive transducer to convert a signal consisting of photons into an easily measured electrical signal. Ideally the detector s signal, S, should be a linear function of the electromagnetic radiation s power, P,... 

Multiplying the transmittance by 100 gives the percent transmittance (%T), which varies between 100% (no absorption) and 0% (complete absorption). All methods of detection, whether the human eye or a modern photoelectric transducer, measure the transmittance of electromagnetic radiation. 

Imaging systems, consisting of specialty chemicals and techniques, are used to produce copies or photographic representations of macroscopic entities that can be seen by the human eye. Moreover, imaging systems are utilized to produce representations of what is outside the range of human vision. 

Eor LEDs utilized in visible/display appHcations, the human eye serves as the detector of radiation. Thus a key measure of performance is luminous efficiency which is weighted to the eye sensitivity (CIE) curve. The relative eye sensitivity, V (L), peaks in the green at A 555 nm where it possesses a value of 1.0. It drops sharply as the wavelength is shifted to the red or blue, reaching a value of 0.5 at 510 and 610 nm. The luminous efficiency, in units of Im/W, of an LED is given by equaton 11 ... 

Pigure 10 shows the typical commercial performance of LEDs used for optical data communication. Both free-space emission and fiber-coupled devices are shown, the latter exhibiting speeds of <10 ns. Typically there exists a tradeoff between speed and power in these devices, however performance has been plotted as a function of wavelength for purposes of clarity. In communication systems, photodetectors (qv) are employed as receivers rather than the human eye, making radiometric power emitted by the devices, or coupled into an optical fiber, an important figure of merit. 

C is the concentration of limiting reactant in mol/L, c is the chemiluminescence quantum yield in ein/mol, and P is a photopic factor that is determined by the sensitivity of the human eye to the spectral distribution of the light. Because the human eye is most responsive to yellow light, where the photopic factor for a yellow fluorescer such as fluorescein can be as high as 0.85, blue or red formulations have inherently lower light capacities. 

The filter and screen of the pyrometer shown ia Figure 9 require specific mention. From equation 21 it is evident that the observed radiation must be limited to a narrow bandwidth. Also, peak intensity does not occur at the same wavelength at different temperatures. The pyrometer is fitted with a filter (usually red) having a sharp cut-off, usually at 620 nm. The human eye is insensitive to fight of wavelength longer than 720 nm. The effective pyrometer wavelength is 655 nm. 

Erosion can be caused by small particles not visible to the human eye, like dissolved minerals in hard water. Larger solids like sand, boiler scale, and rust can also cause serious erosion inside the pump. 

There are two main types of light-sensitive receptor cells in the retina of the human eye, rod cells and cone cells. The 3 million or so rod cells are... 

The electromagnetic spectrum. Note that only a small fraction is visible to the human eye. 

The conversion of radiated power (P in watts) to luminous flux (F in lumens) is achieved by considering the variation with wavelength of the human eye s photopie response. Then the spectral power from the source (PA in, lor example, W/nnt) is convoluted with the relative spectral response of the eye (V tabulated by the CIE) according to ... 

On this planet, of what value would be an eye that is sensitive only to light in the ultraviolet spectral region Discuss the evolutionary significance of the facts that the human eye and the photosynthesis process are both dependent upon light in the part of the spectrum called the visible. ... 

The value of the ratio [InB]/[InA] (i.e. [Basic form]/[Acidic form]) can be determined by a visual colour comparison or, more accurately, by a spectrophotometric method. Both forms of the indicator are present at any hydrogen-ion concentration. It must be realised, however, that the human eye has a limited ability to detect either of two colours when one of them predominates. Experience shows that the solution will appear to have the acid colour, i.e. of InA, when the ratio of [InA] to [InB] is above approximately 10, and the alkaline colour, i.e. of InB, when the ratio of [InB] to [InA] is above approximately 10. Thus only the acid colour will be visible when [InA]/[InB]> 10 the corresponding limit of pH given by equation (5) is ... 

E. Photoelectric photometer method (Section 17.6). In this method the human eye is replaced by a suitable photoelectric cell the latter is employed to afford a direct measure of the light intensity, and hence of the absorption. Instruments incorporating photoelectric cells measure the light absorption and not the colour of the substance for this reason the term photoelectric colorimeters is a misnomer better names are photoelectric comparators, photometers, or, best, absorptiometers. 

The Munsell book standards corresponding to the limiting colors may even serve as material standards for industrial color control. In a material standard system the sample is compared with a standard by eye without the use of any meter or optical instrument. The success and popularity of these systems are largely due to their simplicity of application. The ability of the human eye to compensate for various illuminants and surroundings makes it possible for this system to give results even under mediocre conditions. The most critical work with material standards requires carefully controlled observing conditions. 

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