Spectrometers are optical instruments used to measure electromagnetic radiation properties, e.g., light intensity, over a specific portion of the electromagnetic spectrum range from the infrared band to the gamma ray band. Normally, a particular spectrometer operates over a limited portion of this total range because different techniques are used to measure the energy in different portions of the electromagnetic spectrum. Spectrometers determine the distribution of spectral energy portions within the electromagnetic spectrum by measuring the intensity of radiation absorbed, reflected, or emitted by a material as a function of different wavelengths, separating the radiation measurements into energy bands, and indicating the relative intensities of the energy in each band. Typically, spectrometers use detectors other than photographic film to determine the distribution of radiation in a particular portion of the electromagnetic spectrum. One example of a common detector used in a spectrometer is a charge coupled device (CCD).
Spectrophotometers are spectrometers that measure the ability of a material's surface to reflect spectral energy, i.e., the spectral reflectance of the material. While the measured spectral energy is usually in the visible band of the electromagnetic spectrum, the measured spectral energy may also be in the infrared, ultraviolet, or other bands. Spectrophotometers are commonly used to measure the spectral reflectance of the surfaces of printed matter, textiles, molded and machined parts, etc., for quality control and/or process characterization. A spectrophotometer typically measures integrated spectral reflectance in a small circular area, e.g., an area four to eight millimeters in diameter, thereby providing only one number to represent what may in reality be spectral energy at a plurality of different wavelengths, each of which varies in intensity at different locations within the circular area. Even though four to eight millimeters is a relatively small area, for many surfaces, e.g., textiles, spectral reflectance can vary widely over a surface area of this size. Hence, it is difficult to use a single spectrophotometer measurement to precisely determine how reflected spectral energy varies over a particular surface area.
One way to measure reflected spectral energy whose intensity in different bands varies over a surface is to take multiple, simultaneous samples across a grid of locations on the surface. Preferably, the spacing of the locations in the grid is small enough to provide information about the spatial variation of the surface's spectral reflectance. One way to take multiple, simultaneous, closely spaced samples over a surface is to use a digital camera. Typically, in a digital camera, radiant energy, e.g., light, passes through a lens, an aperture, a color filter array and strikes a two dimensional sensor array that records the radiant energy. The sensors in digital camera sensor arrays are often charge coupled devices (CCDs). A CCD is an integrated circuit containing an array of light-sensitive diodes that convert radiant energy into an electrical charge proportional to the intensity of the radiant energy. Each diode has a capacitor that can accumulate charge over an exposure time. Circuitry external to the CCD array converts the charge in each CCD element to a voltage that is converted to a digital value that is stored in an electronic memory device. Used conventionally, the array of values from the CCD array are viewed as an image with each CCD providing one picture element (pixel) in the image. An alternative to a CCD array is an Active Pixel Sensor (APS). An APS is a complementary metal oxide semiconductor (CMOS) device in which the sensing circuitry is combined with the recording circuitry.