Photocell, barrier layer

Adjustable iris diaphragm 11. Barrier-layer type photocell... 

There are another type of photocells known as barrier layer photocells which work on an entirely different principle. They are semiconductor devices in which impinging photons promote the electrons from the valence band to the conduction band across the energy gap. A photovoltagc is generated which can be measured by a voltmeter. Such photovoltaic devices can have a large surface area and are easy to operate. They are commonly used in many simple colorimeters and fluorimeters and as light Otters for cameras. 

Vacuum phototubes are preferred as detectors over barrier-layer photocells because of their higher signal-to-noise ratio, greater stability, longer life, and freedom from fatigue. Simple tubes are preferred over multiplier types because they are less costly, are more stable, and can be used in simpler circuits. 

A slit adjustable for both length and width is desirable, but improvisation with black tape can compensate for the absence of proper adjusting mechanisms. A slit width of about 1 mm is needed to provide enough light for the classic barrier-layer photocell, but as narrow a slit as possible should be used to minimize the degree of uneveness in the area covered. Toward the edges of the paper strip the protein (and therefore the dye) concentration of the zones decreases (variably because of different diffusion rates) to zero. The length of the slit should be such that only the central, protein-filled part of the paper strip is scanned (Fig. 

The light leaving the paper should pass through an exit slit to avoid the effect of scatter on the surface of the barrier-layer photocell (Fig. 

Some of the workers who published absorption methods for sodium also include data on potassium. While certain differences exist between sodium and potassium in regard to sensitivity and interferences, the equipment used by some authors was of the same simplicity for both. Filters and barrier-layer photocells sufiice, but when using photomultiplier tubes their reduced sensitivity in the red, where the potassium resonance line is located, has to be taken into account. 

Different types of photosensitive detectors have been used for spectral intensity measurements, including barrier layer photocells, vacuum and gas photodiodes, and multiplier phototubes. By far the most commonly used device is the multiplier phototube because of its extremely high sensitivity and precision when powered by a voltage-regulated power supply. A variety of multiplier phototubes are available that have maximum response in different wavelength regions. 

For the first time, there was a mathematical expression for the impedance dispersion corresponding to the circular arc found experimentally. The equation introduced a new parameter the somewhat enigmatic constant a. He interpreted a as a measure of molecular interactions, with no interactions a = 1 (ideal capacitor). Comparison was made with the impedance of a semiconductor diode junction (selenium barrier layer photocell). 

Cannon and Butterworth (1953) demonstrated that a linear plot of Beer s law is no proof of linearity of spectrophotometer response, and erroneous absorbancy values may be obtained. They suggest an instrumental check by use of independently calibrated neutral filters. Their findings apply particularly to the barrier-layer photocells, used in such instruments as the Unicam, the smaller Coleman, the Evelyn, Hunter, Klett-Summerson, and Photovolt, and are less applicable to the gas-filled photocells used in the General Electric Recording and Beckman instruments. Errors in standards for spectrophotometry as well as errors concerned with the General Electric instrument are discussed by Gibson (1949). 

The Hunter tristimulus colorimeter represents a departure from the usual colorimeters, as it presents its data in a form closely akin in spacing to the Munsell system. By resistance networks between a series of barrier-layer photocells which view the reflected light from a specimen through tristimulus filters and a compensating cell which views the light source, a reading is obtained which is related to perceptible differences. 

The use of barrier-layer photocells in some of these instruments introduces other difficulties. As the radiant energy impinging upon the photocell decreases, the current likewise decreases to such an extent that it is impossible to obtain accurate readings at low intensities. In most instruments this occurs at approximately 2% reflection. 

A new instrument for automatic colorimetric and fluorimetric titrations is described. Titrant delivery is automatically stopped at the end point by the use of a microammeter with a built-in relay which stops a motor-driven buret equipped with digital read-out or a gravity driven buret equipped with solenoid control. Monochromatic light is used to excite fluorescence, which is received at 90° by a photomultiplier tube with the microammeter in its anode circuit. Light transmitted through the titration vessel is detected by a barrier-layer photocell, the output of which is fed to the microammeter. 

The optical system is shown in Fig. 1. It consists of a tungsten lamp, a lens to collimate light from the lamp through the titration vessel and onto the barrier-layer photocell,t a lens to image fluorescent light from the titrates onto the photomultiplier tube, and filters to make the light from the lamp and titrate monochromatic. 

The detector and read-out unit depend on whether filters or monochromators are used. With filters a barrier layer photocell and sensitive galvanometer give sufficient sensitivity, whilst with a monochromator photomultipliers are required. 

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