GB2064760A - Light meter - Google Patents

Abstract

A light meter for measuring optical density or reflectivity of an object uses a photocell (1) having a linear response to light intensity connected to an amplifier (2) having selectable feedback loops (3, 4) to vary the sensitivity of the meter. The output is digitalized (5) and then logarithmically converted (6) to provide an indication of optical density or reflectivity which is numerically displayed on a display (7). By proper arrangement of the scaling in the display means, the same light meter can also be used as a halftone dot area meter. <IMAGE>

Description

SPECIFICATION Light meter This invention relates to a light meter which can function either as a photoelectric densitometer for measuring the magnitude of the light transmitted through an object whose optical density is desired to be known or as a photo-electric reflectometer for measuring the magnitude of light reflected from an object whose optical reflectivity is desired to be known. In other words, this invention relates to a light meter which can be used either for measuring the optical density of an object or for measuring the optical reflectivity of an object.

Although the invention relates to both the above mentioned types of light meters, their essential features are identical with the only difference arising from the different manners of application. Therefore, in the following description, attention is directed only to one of the two types of light meters; viz. a photoelectric densitometer, which is for measuring the optical density of an object, especially photographic film.

Normally, a photoelectric densitometer is required to have a measuring range or a dynamic range which extends from 0.0 to 4.0 for most practical purposes. The range of optical density unit amounts to from 1.0 to 0.0001 in terms of the ratio of the magnitudes of the transmitted light and the incident light. And the resolution of a photoelectric densitometer is desired to be better than 0.002 in terms of the light ratio since a gradation of at least 0.01 is desired to be detected.

In general, the following relation holds between optical density D and transmittance T, the latter corresponding to the ratio of the light magnitude before and after the transmission: D = log10(1/T) As a requisite function of a photoelectric densitometer, an electric signal corresponding to transmittance T must be obtained first of all and then the signal must be logarithmically converted to be displayed numerically on an appropriate display device.

As conventional photoelectric densitometers, there have been available two types, one using a phototube or a photomultiplier tube and the other using a semiconductor light sensor.

The former type of photoelectric densitometers, normally, the electric signal obtained from a phototube or photomultiplier as the case may be is logarithmically converted in a special circuit in which an approximately logarithmic function is created. However, since such photoelectric densitometers tend to be bulky and normally require a high voltage source, the latter type of photoelectric density meters have come to be more widely used.

With regards to the latter type of photoelectric densitometers, there are a variety of semiconductor light sensors, which can be used in photoelectric densitometers, available in the market. They may be generally categorized into photoconductive cells which change their internal resistances according to the magnitude of the light to which they are exposed to and photocells which generate electric voltage or current according to the magnitude of the light to which they are exposed to.

Any of such light sensors can be used in photoelectric densitometers but there are some suitable types and unsuitable types depending on the area of application. But such selection of light sensors is a matter of design choice and a detailed discussion of the subject is not given here. And, it is obvious to a person skilled in the art, a minor change of circuitry becomes necessary when changing the type of light sensor, when using different properties of a light sensor, or when using a different operation condition for a same light sonsor.

When making a photoelectric densitometer using a semiconductor light sensor, it is normally possible to use either a logarithmic property of the sensor or a linear property of the light sensor. For example, a photocell consisting of a silicon PN-junction can either generate voltage logarithmically proportional to the light to which it is exposed by connecting the device to a high-impedance input, or, alternatively, generate electric current which is linearly proportional to the magnitude of the light to which the device is exposed to by connecting it to an extremely low impedance input.

According to the former approach, though the circuitry may be considerably simplified as there is no need to convert the signal logarithmically, it becomes very difficult to eliminate the influences of temperature on the output voltage. Also, it is difficult to obtain a satisfactory wide range of operation.

However, according to the latter approach, the cost for manufacture tends to be high since the obtained voltage has to be converted logarithmically and there is some difficulty involved in obtaining a satisfactory temperature properties and an agreeably wide operating range.

In view of the shortcomings of conventional photoelectric densitometers, one of the primary object of this invention is to provide a new photoelectric densitometer which is satisfactory both in temperature properties and precision and yet economical for manufacture, using the short-circuit current of a semiconductor light sensor, or connecting the sensor to an extremely low input impedance, and converting the obtained short-circuit current logarithmically with a digital device.

According to this invention, such an object is accomplished by a photoelectric densitometer which comprises a semiconductor light sensor which detects the intensity of light, an operational amplifier which receives the shortcircuit current of the light sensor and converts the current into a voltage, a switching circuit, connected in parallel with the operational amplifier, comprising a plurality of feedback resistors, each connected in series with a selective switch whose internal resistance is negligibly small with respect to the feedback resistor connected in series thereto, an analogdigital converter for converting the output voltage of the operational amplifier into a digital signal, an operational circuit for converting logarithmically the digital signal, and a display for numerically displaying the output from the operational circuit.

Also, according to this invention, it is possible to adapt the photelectric densitometer for use as a halftone dot area meter by properly adapting the scale factor and the zero point.

In the following, this invention is described in detail with respect to embodiments of this invention making reference to the sole figure of the appended drawing, in which the circuitry of a preferred embodiment of this invention is shown in a block diagram.

As shown in the drawing, the output from light sensor 1, which is a silicon photocell and produces an electric current which is proportional to the magnitude of the light directed upon the sensor, normally over a range of 105, iS given to operational amplifier 2 to be converted into a corresponding electric voltage signal.

To operational amplifier 2 is connected a feedback circuit, in parallel, which consists of a plurality of feedback resistors 3a, 3b, 3c and 3d, each connceted in series with one of analog switches 4a, 4b, 4c and 4d. These analog switches are preferred to have internal resistances negligibly smaller than the feedback resistors connected in series thereto.

The resistances of feedback resistors 3a, 3b, 3c and 3d are selected such that (3a)x 1,000=(3b)X 100 = (3c)X 10 = (3d) where the numerals in the brackets indicate the resistances of the feedback resistors to which the numerals pertain. Therefore, by selectively operating analog switches 4a, 4b, 4c and 4d, it is possible to obtain a currentvoltage conversion which can be expressed as; Ego = I,Ri where E0 is the output voltage of the operational amplifier circuit, 15 is the short-circuit current produced from the light sensor, and R1 is the resistance of a resistor selected from feedback resistors 3a, 3b, 3c and 3d.

The output voltage of operational amplifier 2 is then given to 8-bit analog-digital converter 5 and converted into an 8-bit digital signal. The digital signal is then sent to operational circuit 6, which is essentially a micro computer, where the digital signal is logarithmically converted using an appropriate functional approximation, and displayed on display 7.

Since analog-digital converter 5 of this preferred embodiment is of an 8-bit type, the range of output is from zero to 255 according to the decimal system.

Therefore, when optical density is zero, the magnitude of the illuminating light or light sensor 1 is adjusted to obtain 250 from the output of analog-digital conveter 5 selecting resistor 3a as the feedback resistor to be connected. Thus, with the feedback resistor selected at resistor 3a, the output of analogdigital converter 5 ranges from 250 to 25 while optical density varies from 0.0 to 1.0.

With resistor 3b selected as the feedback resistor for operational amplifier 2 increasing the gain of the amplifier by ten times, then the output of analog-digital converter 5 changes from 250 to 25 as optical density is increased from 1.0 to 2.0. Likewise, optical density ranges from 2.0 to 3.0 and from 3.0 to 4.0 are reduced to the range of the output from 250 to 25 by selecting the feedback resistors to resistors 3c and 3d, respectively.

Thus, the whole range of optical density from 0 to 4.0, or from 1.0 to 0.0001 in terms of the ratio of light magnitude, is covered by the output range of analog-digital converter 5, from 250 to 25 merely by selecting one of the four feedback resistors.

Therefore, as opposed to the case where no such selection is made, influences of off-set voltage of the operational amplifier on the output can be substantially reduced. Furthermore, requirements on analog-digital converter 5 with regards to the capability of resolution can be considerably reduced.

For example, if one tried to measure the ratio of light magnitudes from 1.0 to 0.0001 without selecting the gain of the operational amplifier, it would be necessary to have a 20bit analog-digital converter in order to have any sort of resolution. If a resolution of 0.002 in terms of the ratio of light magnitude is required, an impractically large number of bits become necessary in the analog-digital converter.

In the case of the above-described embodiment, optical density D can be related to the output of analog-digital converter 5 r by the following formula: D=log,,(250/r) As can be readily seen, the output of analog-digital converter 5 ranges from 250 to 25 as optical density changes from 0.0 to 1.0 with the feedback resistor selected at resistor 3a. When resistors 3b, 3c and 3d are used as the feedback resistor the resulting ranges of optical density are from 1.0 to 2.0, from 2.0 to 3.0, and from 3.0 to 4.0, respectively.

Although the ratio of the resistances of the feedback resistors was ten, one resistance over another, but it is obvious to a person skilled in the art that the ratio can be arbitrarily selected according to one's need. For example, the ratio may be selected at five and a 10-bit analog digital converter can be used instead of the 8-bit converter, thereby increasing the resolution of the resulted photoelectric densitometer.

As a further application of the photoelectric densitometer of this invention, it can also be used as a halftone dot area meter simply by properly adjusting the scaling of the display.

Halftone dot area ratio A is the ratio of the area which is occupied by halftone dots as opposed to blank areas. Halftone area ratio A can be related to the output of the analogdigital converter by the following formula: A = [(M - M3 Mz)] X M31 x 100 (%) where M = 100 X (1 - r/250), r being the output of the analog-digital converter, while MZ and MH are the values of M when halftone dot area ratios are 0% and 100%, respectively.

It is preferable to have an arrangement in a photoelectric densitometer such that the operator may be able to adjust the densitometer to a halftone dot area meter by establishing the above-described relation on the meter.

Claims (10)

1. A light meter, comprising: a semiconductor light sensor which detects the intensity of light; an operational amplifier which receives the short-circuit current from the light sensor and converts the current into a voltage: a switching circuit, connected in parallel with the operational amplifier, comprising a plurality of feedback resistors, each connected in series with a selective switch whose internal resistance is negligibly small with respect to the resistance of the feedback resisitor connected in series thereto; an analog-digital converter for converting the output voltage of the operational amplifier into a digital signal, an operational circuit for converting logarithmically the digital signal; and a display for numerically displaying the output from the operational circuit.

2. A light meter according to claim 1, wherein the light meter is a photoelectric densitometer which detects light transmitted through an object whose optical density is desired to be known.

3. A light meter according to claim 1, wherein the light meter is a photoelectric reflectometer which detects light reflected from an object whose optical reflectivity is desired to be known.

4. A light meter according to claim 2 or 3, further comprising an arrangement by which the light meter can be adapted to be a halftone dot area ratio meter with a proper conversion of the scaling of the display.

5. A light meter according to claim 4, wherein the light sensor is a silicon photocell comprising a PN-junction.

6. A light meter according to claim 5, wherein each of the switches is an analog switch.

7. A light meter according to claim 6, wherein the operational circuit is a micro digital computer in which the logarithmic function is created by a numerical process.

8. A light meter according to claim 8, wherein the analog digital converter is an 8bit device.

9. A light meter according to claim 9, wherein the feedback resistors are selected by analog switches in four stages.

10. A light meter substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.