High sensitivity light absorbance apparatus

Claim 2

Apparatus for measuring light intensity comprising: photoresistive means for developing a first signal that is substantially proportional to the intensity of light incident upon the photoresistive means; logarithmic means, electrically connected in circuit with said photoresistive means, for developing a second signal that is substantially proportional to the logarithm of the first signal; output means adapted to provide said second signal to a utilization unit, whereby said utilization unit receives information concerning said light intensity; and operational amplifier means electrically connected by a linear direct signal path to said photoresistive means and in circuit with said logarithmic means, to provide a load resistance to said photoresistive means that is very small compared to the resistance of said photoresistive means and to control the potential across said photoresistive means and logarithmic means as the resistance of said photoresistive means varies to provide precision, fast response, low noise and stability to said apparatus in supplying said second signal to said output means.

Claim 3

Apparatus according to claim 1 in which said operational amplifier means comprises: an operational amplifier having an operational amplifier input terminal and an operational amplifier output terminal; said operational amplifier input terminal being electrically connected to said photoresistive means; said operational amplifier output terminal being electrically connected to said logarithmic means anD said output means, whereby said output means receives said second signal; said operational amplifier means further comprising a feedback path between said operational amplifier output terminal and said operational amplifier input terminal, whereby compensation is provided for the voltage drop across said logarithmic means so that the precision, stability, speed of response and noise characteristics of said apparatus are improved.

Claim 4

Apparatus, according to claim 2 in which said logarithmic means is electrically connected to said operational amplifier output terminal through an output impedance element having a greater impedance than said logarithmic means and said output means is electrically connected between the output impedance element and the logarithmic means.

Claim 5

Apparatus according to claim 2 in which the logarithmic means is a logarithmic diode.

Claim 6

Apparatus according to claim 2 in which said feedback path includes said logarithmic means and a feedback resistance means connected electrically in series.

Claim 7

Apparatus according to claim 5 in which: said output means includes an output resistor and an output terminal electrically connected in parallel at a junction between said logarithmic means and said feedback resistance means; said feedback resistance means having greater resistance than said output resistor, whereby said second signal is developed as a potential across said logarithmic means.

Claim 8

Apparatus according to claim 5 in which: said photoresistive means includes first and second photoresistors, each having different first and second photoresistor terminals; said first photoresistor terminals of said first and second photoresistors being adapted to be electrically connected to a source of potential; said operational amplifier having a second operational amplifier input terminal; the second photoresistor terminal of one of said first and second photoresistors being electrically connected to said first-mentioned operational amplifier input terminal; the second photoresistor terminal of the other of said first and second photoresistors being electrically connected to said second operational amplifier input terminal; one of said first-mentioned and second operational amplifier input terminals being an inverting terminal, whereby changes in the potential across one of said first and second photoresistors results in compensating changes in the potential across the other of said first and second photoresistors from the feedback path of said operational amplifier means to stabilize, reduce the noise and increase the speed of response of said apparatus.

Claim 9

Apparatus according to claim 7 in which: said logarithmic means includes a plurality of logarithmic diodes; at least one of said plurality of logarithmic diodes being electrically connected between said operational amplifier output terminal and said first-mentioned operational amplifier input terminal; at least one other of said plurality of logarithmic diodes being electrically connected to said second operational amplifier input terminal; said output means being electrically connected to said operational amplifier output terminal, whereby a bridge is formed for comparing the resistances of said first and second photoresistors and for balancing the changes in said one and one other of said plurality of logarithmic diodes that occur due to temperature changes and for balancing the changes in potential across the one and one other of said plurality of logarithmic diodes that occur because of changes in the resistances of the photoresistors so as to improve the precision with which the apparatus operates.

Claim 10

Apparatus according to claim 8 in which said feedback resistance means includes at least one variable resistor connected electrically in series with one of said one and one other of said plurality of logarithmic diodes, whereby the response of said apparatus may be adjusted.

Claim 11

Apparatus according to claim 8 in which at least two of said plurality of logarithmic diodes are electrically connected between said operational amplifier output terminal and said first-mentioned operational amplifier input terminal and at least two different diodes of said plurality of logarithmic diodes are electrically connected between said operational amplifier output terminal and said second operational amplifier input terminal.

Claim 12

Apparatus according to claim 8 further including means for developing the anti-logarithm of the second signal and for providing an output voltage which varies in correspondence with the ratio of the percent transmittances with respect to the intensity of the light reaching said first and second photoresistors.