This Photometer consists of a metal box with two entrances and an eyepiece as well as a slot which allows a white magnesium carbonate disk to be mounted between the two entrances. Similar to the Weber Photometer this item was designed to compare the brightness of an unknown light source to the brightness of a standard light source. Light from each of the two sources enters through its respective entrance and illuminates one side of the white disk. The light that is reflected off each side of the disk is directed through a right angled prism and towards a Lummer-Brodhun cube. This cube directs the light rays coming from both sides of the disk into the eyepiece so that the user can observe the light from both sources simultaneously.
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A photometer is an instrument which can be used for comparing the luminous intensities of sources of light. Lamps of luminous intensities I1 and 12 respectively are placed on opposite sides of a white opaque screen, and some of the diffusely reflected light from the opposite surfaces A, B is incident on two identical totally reflecting prisms P,O,. This consists of two right-angled isosceles prisms. In optical contact at their central portion C, but with the edges of one cut away so that an air-film exists at M, N all round C between the prisms.
By moving one of the sources, however, a position is obtained when both portions appear equally bright, in which case they cannot be distinguished from each other and the field of view is uniformly bright. Suppose that the distances of the sources I1 and I from the screen are d1, d respectively.
Hence, for a photometric balance. The ratio of the intensities are hence proportional to the squares of the corresponding distances of the sources from the screen. The reflection factors r1, r are not likely to be exactly equal, however, in which case another or auxiliary lamp is required to compare the candle-power I1, I. The auxiliary lamp I2, is placed on the right side, say, of the screen at a distance d2, and one of the other lamps is placed on the other side.
A photometric balance is then obtained, The figures above compares Luminous intensities In this case figure 1 , if I1 is the intensity of the lamp and d1 is its distance from the section, The remaining lamp I is then used instead of the lamp I1, and a photometric balance is again obtained by moving this lamp, keeping the position of the lamp I2 unaltered, Figure 2. Suppose the distance of the lamp I from the screen is d.
The intensities I1, I are hence proportional to the squares of the corresponding lamp distances from the screen. Measurement of Illumination It was pointed out at the beginning of the chapter that the maintenance of standards of illumination plays an important part in safeguarding our health.
It is recommended that desks in class-rooms and offices should have an illumination of lux, and workshop an illumination of lux; for sewing dark materials an intensity of — lux is recommended, while lux is suggested for the operating table in a hospital. A modern type is the photovoltaic cell, which may consist of a cuprous oxide and copper plate, made by, Oxidizing one side of a copper disc D, See Figure above.
When the oxide surface is illuminated, electrons are emitted from the surface whose number is proportional to the incident luminous energy, and a current flows in the microammeter or sensitive moving-coil galvanometer G which is proportional to the illumination. The galvanometer is previously calibrated by placing a standard lamp at known distances from the disc D, and its scale reads lux directly.
112 - Lummer- Brodhun Photometer