Illuminant Discriminator

The present invention relates to an apparatus for discriminating among various types of light sources, such as fluorescent light, incandenscent light and natural daylight.

To produce faithful photographic reproductions of multicolored scenes, the color balance of the photographic film must be compatible with the spectral characteristics of the scene illuminant. Many photographic color emulsions are color balanced for use with natural daylight and others are color balanced for use with incandescent illumination. To properly expose a color film with an illuminant for which the film is not color balanced it is necessary to use color compensating filters.

A patent of interest for its teaching of a method and apparatus for discriminating illuminant light in U.S. Pat. No. 4,220,412 entitled "Illuminant Discrimination Apparatus and Method" by R. A. Shroyer et al. The method and apparatus disclosed in that patent utilizes the temporal signatures of the various light components based upon the harmonical distorted sine wave signal that is derived from the illuminant source impinging on a photodiode. The photodiode produces an electrical signal having an amplitude which varies with the instantaneous intensity of the illuminant. A means is provided for detecting the amount of harmonic distortion in the signal and for indicating the type of illumination impinging on the photodiode as a function of the distortion. In addition, the apparatus is combined with flicker ratio detecting circuitry to provide a system which is capable of discriminating between fluorescent light, incandescent light and natural daylight. The flicker ratio is the ratio of the brightest to the dimmest intensities of the light during a given time interval. Natural light, like other light emanating from a source of constant brightness, has a flicker ratio of unity. Artificial light sources, being energized by ordinary household line voltage, have a brightness which flickers at approximately 120 hertz, twice the frequency of the line voltage. Owing to the different rates at which the energy-responsive elements of incandescent and fluorescent lamps respond to applied energy, such illuminance can be readily distinguished by their respective flicker ratio.

With improvements in the state of the art it is highly desirable to have a system that is heavily digitized for performing the illumination detection. Such systems are readily manufactured incorporating integrated circuits for size and cost reductions. It is highly desirable to keep the use of analog circuits to a minimum as analog circuits are more difficult to implement. In addition, signal processing of the type used with this invention is more quickly accomplished with digital circuitry. In camera applications, the illuminant light source has to be determined almost simultaneously with the pressing of the shutter button in order to provide meaningful aperture adjustment information.

Because illumination takes the form of a periodic wave other methods of deriving signal equivalents are suggested. One patent of interest for its handling of the analysis of periodic waveforms is, U.S. Pat. No. 4,301,404 entitled "Methods and Apparatus for Analyzing Periodic WaveForms" by A. J. Ley. In that patent, a periodic waveform is repetitively sampled at an integer multiple of the frequency of the waveform over a number of cycles and a sum is formed for each sample with the corresponding sample in the previous cycles. A Fourier transformation is applied to the summed samples to derive a measurement of the component corresponding to the summed samples and to the waveform in general.

Another patent of interest for its teaching is, U.S. Pat. No. 4,296,374 entitled "Wide Band Digital Spectrometer" by P. S. Henry. In that patent, the inventor utilizes a Walsh Fourier transformation to perform an analysis of a periodic waveform. The disclosed apparatus performs a wideband digital spectrometry utilizing Walsh functions to achieve a simplified method of producing the Fourier power spectrum of an input signal.

The present invention provides an apparatus and an associated method which is capable of determining the type of illuminant light that is impinging on the apparatus. The apparatus is comprised of two major portions, an analog portion for conditioning an impinging illuminant signal so that it can be readily converted into a digital signal by an analog-to-digital converter and a second portion which incorporates an analog-to-digital converter and a means for processing the digital signal by performing a Fourier series analysis on the digital signal.

The analog portion of the apparatus is comprised of, a photodiode sensor for transforming the impinging illuminant signal into an AC electrical signal, a transimpedance amplifier, a logarithmic amplifier for processing the AC signal, and a DC blocking shifting stage for providing a level shifted AC signal which is a function of the received illuminant signal.

The digital portion of the apparatus is comprised of an analog-to-digital converter that is coupled to a microprocessor. The microprocessor performs the Fourier series analysis on the converted AC signal by determining, for one or more harmonics of the illuminant signal, the amplitudes of a multiplicity of the harmonics of the converted AC signal and by comparing the determined amplitudes against the threshold values of known illuminant sources.

From the foregoing it can be seen that it is a primary object of the present invention to provide an improved illuminant discrimination apparatus.

It is a further object of the present invention to provide an apparatus for identifying illumination sources which apparatus is conducive to digital implementation.

These and other objects of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein like characters indicate like parts and which drawings form a part of the present specification.

FIG. 1 illustrates, in schematic diagram form, the analog portion of the present invention.

FIG. 2 illustrates, in block diagram form, the digital portion of the present invention.

Referring to FIG. 1 a photodiode 12 converts an impinging illuminant light beam into a corresponding electrical signal which is coupled to an input of a differential amplifier 26A. In the preferred embodiment, the photodiode 12 is an NSL-710BG device and the amplifiers 26A-26D are incorporated in a TL084 device both devices being manufactured by National Semiconductor Inc. The illuminant light source may be tungsten, daylight and/or fluorescent light or a combination of these. The discriminator functions to identify which is the dominant source. The photodiode 12 is connected in circuit to form part of a transimpedance amplifier stage 10. The output signal from the amplifier stage 10 is directed to a log amplifier stage 15. The transimpedance amplifier stage is used to amplify the signal from the photodiode and the log amplifier stage is used to compress the range of signals that are passed onto the remainder of the circuitry. The output signal from the log amplifier stage is directed to a level shift and gain stage 20, via a DC blocking capacitor 22. A biasing network 24 comprised of a 100K potentiometer coupled between a +9 V and -9 V source biases the positive input of the differential amplifier 26C so as to provide the proper level shift to the AC signal component present on its negative input. The proper level shift is achieved with approximately +2.5 V on the positive input of amplifier 26C. The output is thus adjusted to range between 0 and +5 V. The level shifted output signal from the amplifier 26C is further amplified by the gain stage 25, which incorporates an amplifier 26D. The output signal from the amplifier 26D is a discriminator signal which is the input to the analog-to-digital, A/D, portion of the present apparatus, shown in FIG. 2. The discriminate signal is clamped to a reference potential by means of a diode clamp IN914 and a voltage divider network 27. The voltage divider network 27 is comprised of a 100K potentiometer coupled between a +9 V and a -9 V source.

The output signal from amplifier 26D is clipped at 0 volts and ranges to +5 V. The voltage divider network 27 is adjusted to provide approximately -0.7 volts to the IN914 diode.

Referring now to FIG. 2 wherein is shown the digital portion of the present invention. An A/D converter 50 receives on its input, labeled 7, the discriminator signal from the output of the gain stage 25 and operates upon the analog discriminator signal to provide a corresponding 8-bit digital signal on its outputs labeled DB0 to DB7. The A/D converter 50 may be a National Semiconductor device bearing the part number ADC0801. The 8-bit digitized signal is then directed to a microprocessor 60 which may be a microcontroller of the type manufactured by Intel Corporation under their part number D8751H. In the preferred embodiment of the invention the microprocessor 60 analyzes the digital signals appearing on its PD.0 to PD0.7 inputs and provides at three independent outputs, P3.0, P3.1 and P3.2, an indication, via the associated LED devices, as to which one of the three types of illuminating light is dominantly impinging on the photodiode 12 of FIG. 1. If the LED device connected to the output P3.0 is lit, the illuminant source is tungsten. If the LED device connected to the output P3.1 is lit, the illuminant source is daylight and, if the LED connected to the output P3.2 is lit, the illuminant source is fluorescent.

The microprocessor 60 operates upon the fundamental frequency component and one or more harmonical components of the digitized input signal and determines from evenly spaced samples the fundamental frequency and the first harmonic. The amplitudes of each frequency are determined by the square root of the squares of the sine and cosine terms.

The above is derived directly from the following formula: ##EQU1## where: k=normalized frequency

n=sample number

N=number of samples/cycle

The fundamental frequency (120 Hz) component and one or more harmonics ae determined from evenly spaced samples of the input waveform. For example, the fundamental frequency and the first harmonic can be determined from 8 samples spaced 1.042 ms apart, assuming 60 Hz line frequency. Strictly speaking, the amplitudes of each frequency are determined by the square root of the squares of the sine and cosine terms. However, since the number is only being compared to a threshold value nonlinear ranging can be used and numbers representing the values can be determined as follows:

where sn refers to the sample number.

The effective amplification of the second harmonic is unnecessary but the method provides a higher threshold value and a reduction in noise of the weaker amplitude signals. This algorithm is simpler and faster to perform than conventional digital signal processing.

After determining the amplitudes of a sufficient number of harmonics, these amplitudes can be compared against the threshold values of a known light source for identification.

The code used in the microprocessor 60 to perform the analysis and the comparison is set out in the attached Appendix A.

While there has been shown what is considered to be the preferred embodiment of the present invention, it will be manifest that many changes and modifications may be made therein without departing from the essential spirit of the invention. It is intended, therefore, in the annexed claims, to cover all such changes and modifications as may fall within the true scope of the invention. ##SPC1##