LIGHT QUANTITY ADJUSTING DEVICE

PURPOSE:To obtain a fixed light quantity even in the case that the light quantity of a lamp is decreased by replacing an ND filter having low transmissivity with the one having high transmissivity every time the light quantity of the lamp is attenuated by a certain value. CONSTITUTION:As to light from the lamp 2, an infrared ray is allowed to escape by a cold mirror 6 so that only visible light is reflected and further the infrared ray and an ultraviolet ray leaked on the mirror 6 are removed by a UV-IR cut filter 8. The light passes through the ND filter 40 to attenuate its light quantity, and it is color-separated to three primary colors by a color separation dichroic mirror 20 and a total reflection mirror 24 and made incident on three liquid crystal panels 28. At such a time, the light is polarized by a polarizing plate 26, and the light passing through the panel 28 passes through the polarizing plate 26 again, and the video of three colors is reflected on the total reflection mirrors 32 and 34 and projected on a transmission type screen 36 by a projection lens 30. When the light quantity of the lamp is attenuated to a certain value by lighting the lamp 2 for a long time, the lamp using time is detected by a timer and a gear is rotated by a motor, then a detection hole is detected by a photosensor to switch the filter to the filter 40 whose transmissivity is one rank higher.