Comparison photometer and target therefor

Claim 1

IN A COMPARISON PHOTOMETER, THE COMBINATION OF: (A) A FIRST LIGHT SOURCE DEFINING A BACKGROUND AREA OF SUBSTANTIALLY UNIFORM ILLUMINATION; (B) TARGET MEANS DEFINING A REFLECTIVE PATTERN PARTIALLY COVERING SAID BACKGROUND AREA, AND AN INDEX SYMBOL SUPERPOSED ON SAID PATTERN AND HAVING PORTIONS OVERLYING BOTH SAID BACKGROUND AREA AND SAID REFLECTIVE PATTERN; AND (C) A SECOND LIGHT SOURCE FOR ILLUMINATING SAID PATTERN; (D) SAID PATTERN AND INDEX SYMBOL BEING ARRANGED SO THAT THE LATTER IS UNIQUELY INTELLIGIBLE ONLY WHEN THE LEVEL OF BRIGHTNESS OF SAID BACKGROUND AREA BEARS A PREDETERMINED RELATIONSHIP TO THE LEVEL OF BRIGHTNESS OF SAID PATTERN; (E) SAID INDEX SYMBOL BEING DEFINED BY A FIRST PLURALITY OF AREAS THAT ABSORB SUBSTANTIALLY ALL OF THE LIGHT INCIDENT THEREON AND A SECOND PLURALITY OF AREAS THAT TRANSMIT SUBSTANTIALLY ALL OF THE LIGHT INCIDENT THEREON SUCH THAT ALL PORTIONS OF SAID INDEX SYMBOL APPEAR SUBSTANTIALLY UNIFORMLY BRIGHT WHEN THE LEVEL OF BRIGHTNESS OF SAID BACKGROUND AREA BEARS SAID PREDETERMINED RELATIONSHIP TO THE LEVEL OF BRIGHTNESS OF SAID PATTERN.