Transmittance densitometer

In a transmittance densitometer comprising an annular sample stage with a central measuring light path passing centrally therethrough which is illuminated interiorly by illumination light, and a vertically movable ambient light shielding cylinder disposed above the sample stage and along the extension of the measuring light path and surrounding a light path communicating in extension with the measuring light path, the improvement which comprises

• an annular illumination plate made of a semi-transparent substance disposed centrally inside of said sample stage coaxially therewith annularly surrounding the measuring light path and capable of diffusing transmission light,
• an internal light shielding box defining a light shielding cylinder and disposed beneath said illumination plate forming a local illumination chamber optically isolated from the illumination light in the measuring light path and the sample stage,
• an electric switch means for responding to the level of the ambient light shielding cylinder relative to the sample stage, and
• an illumination lamp disposed in said local illumination chamber and operatively connected to said electric switch means for being turned off by said electric switch means when said ambient light shielding cylinder is moved so as to be close to said sample stage.