Apparatus and method for measuring light transmittance or reflectance

In scanning apparatus for taking transmittance or reflectance measurements from a radiation transmitting or reflecting sheet mounted in an operative position so that the sheet can be exposed for being scanned along a line by a radiation beam comprising:

• means for directing a beam of radiation onto the sheet and along a line in scanning relationship thereto when the sheet is in said operative position;
• means defining an elongated cavity having a radiation-receiving slot therein to allow radiation from the line scanning across the sheet to pass into the cavity, the cavity defining means having a radiation-reflecting surface to allow the radiation entering the cavity to be reflected;
• photodetector means for sensing at the ends of the cavity the radiation reflected in the cavity and for generating an electronic signal corresponding to the sensed radiation;
• and
• circuit means coupled with the photodetector means for correcting the signal for variations in the intensity of the radiation entering the cavity as a function of the distance along the slot during scanning of the beam.

A method of taking radiation measurements from a light transmitting or reflecting sheet comprising:

• exposing the sheet for being scanned along a line by a radiation beam;
• directing a beam of radiation onto the sheet and along a line in scanning relationship thereto when the sheet is in the operative position;
• collecting radiation from the line across the sheet within a cylindrical region and directing the collected light to a predetermined location in said region;
• generating an electronic signal corresponding to the collected radation;
• correcting the signal for variations in the radiation entering said cylindrical region as a function of the distance along the slot during scanning of the beam and;
• repeating the directing, collecting, generating and correcting steps until at least a part of the area of the sheet has been scanned by the beam.