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.
|Country:||United Arab Emirates|
|Published (Last):||11 June 2013|
|PDF File Size:||7.86 Mb|
|ePub File Size:||13.21 Mb|
|Price:||Free* [*Free Regsitration Required]|
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. A Lummer-Brodhun cube is made up of two right-angled prisms that are placed with their hypotenuse sides together to form a cube.
Where the prisms do not touch, total internal reflection occurs. Where they do, all the light is transmitted to the other side. The observer can then see a circular disc of light from one source, surrounded by an halo from the other.
By matching the brightnesses of the two, the observer can use the known luminosity of one source and the distances to both sources to calculate the luminosity of the unknown source.
112 - Lummer- Brodhun Photometer