Measurement light meters

Fig. A.3. Light meter used by the author (Model 8020, Pelagic Electronics). For total light measurements, the signals are integrated with capacitors. Milliammeter reading is automatically reset at full-scale position, and the number of resets is digitally indicated below the meter. The box at the right contains a photomultiplier and sample compartment.

In the work with cottonseed flours, we used the Hunterlab color meter D25D2A and expressed these measurements as Hunter L, a, b color values. These are coordinates of the three-dimensional opponent-color space shown in Figure 2. The L value measures lightness, or the amount of light reflected or transmitted by the object. The a and b values are the chromaticity coordinates from which information about hue and saturation can be obtained. The a value measures redness when plus and greenness when minus. The b value measures yellowness when plus and blueness when minus. 

Transmission and Haze Measurements. Light transmission measurements were made with a Gardner color difference meter and the haze measurements, using a Hazemeter attachment on the Gardner colorimeter. Test pieces were 3 X 3 X % inch sheets having glossy surfaces obtained by molding on chrome plates. All measurements were made at 23°C. 

The world is its laboratory, and all those that inhabit this planet may use and enjoy its benefits. Its wave length covers the entire scale of the spectrum. It is a force that cannot be measured in meters or tons and can be transmitted with the speed of light to extreme corners of the earth, using an ether wave as medium. 

Three common classes of instruments are used for measuring the amount of incident light (1) photometers or light meters, which measure the available illuminating power, a quantity related to the wavelength sensitivity of the human eye (2) quantum or photon meters, which measure the number of photons and (3) radiometers, which measure the total energy of the radiation (Fig. 4-4). 

Also added in this draft was a reference on how the VIS exposure should be measured. For Option 1, it was suggested that "White fluorescent lamp (Option 1)—measure light intensity with a lux meter."... 

The extinction coefficient 17 may be calculated from measurements of I made by a light meter. The Secchi disk is another time-honored device for estimating light penetration in a lake. In 1865, P. A. Secchi developed this method, which was subsequently used aboard the papal yacht, S. S. L immacolata Concezione. The Secchi disk is about 20 cm in diameter, col-... 

If radiation of a suitable wavelength falls on a semiconductor it will excite electrons across the band gap. The most obvious effect of this is that the conductivity of the material increases. The magnitude of the effect is roughly proportional to the light intensity. This effect, called the photoconductive effect, has been used in light meters, exposure meters and automatic shutters in cameras and many other devices. In practice, a dc voltage is applied to the ends of a semiconductor. On illumination the resistance of the semiconductor falls, which provides a means of measurement. 

Figure 3 Three basic designs of turbidity meter (A) the nephelometer, which directly measures light scattered (usually at 90° to the beam direction) by suspended particles (B) the turbidimeter, where the transmitted light is detected, in relation to initial beam intensity (C) the ratio turbidimeter in which both transmitted and scattered light is detected. (Reproduced with permission from Hach CC, Vanous RD, and Heer JM (1982) Understanding turbidity measurement. Technical Information Series, Booklet No. 11,1st edn., 11pp. Hach Chemical Co.)...

The human eye is unreliable as an indicator of how much light is present. For accurate results in the measurement of the illuminance at a surface it is necessary to use a reliable instrument. Light meters are available for this purpose. 

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