A spectrophotometer is a precision optical measurement instrument that analyses the spectral composition of light reflected from or transmitted through a coloured sample by measuring the intensity of the light at many individual wavelengths across the visible spectrum - and often into the adjacent ultraviolet and near infrared regions - providing a complete and detailed spectral profile of the sample's colour that contains far more information than the simple tristimulus colour values measured by less sophisticated colorimetric devices. By capturing colour as a full spectral power distribution rather than as a reduced set of three primary colour coordinates, the spectrophotometer provides the most accurate, complete, and device independent description of colour achievable with practical measurement technology, making it the instrument of choice for the most demanding colour measurement, specification, and management applications in professional photography, printing, and colour science.
The operating principle of a spectrophotometer involves illuminating the sample with a known, controlled light source and measuring the intensity of the light reflected or transmitted by the sample at each of a series of narrow wavelength intervals - typically between 10 and 20 nanometres in width - spanning the measurement range of the instrument. The ratio of the reflected or transmitted intensity at each wavelength to the incident illumination intensity at the same wavelength gives the spectral reflectance or transmittance of the sample at that wavelength, and the complete set of these ratios across the full measurement range constitutes the spectral curve of the sample. This spectral curve is the fundamental, device independent description of the colour of the sample, expressing how it interacts with light of each wavelength independently of any particular observer, illuminant, or output device.
From the measured spectral data, the spectrophotometer's software can calculate colorimetric values in any standard colour space - including CIE XYZ, CIELAB, and others - under any specified illuminant and for any specified standard observer, enabling the comparison and communication of colour in a universally understood and reproducible form that is independent of the specific characteristics of any display, printing, or imaging device. This ability to calculate colorimetric values under different illuminants from a single spectral measurement is particularly valuable in applications involving the prediction of metamerism - the phenomenon in which two colours that appear identical under one illuminant appear different under another because their spectral compositions differ despite producing the same tristimulus response for the first illuminant.
In the context of digital photography and colour managed printing workflows, spectrophotometers are used primarily as the measurement instruments for profiling and calibrating the colour behaviour of monitors, printers, and other output devices. When creating an ICC colour profile for an inkjet printer - the mathematical description of how the printer reproduces colours that allows a colour management system to translate image colours accurately from the image file to the printed output - a test chart of known colour patches is printed and measured with a spectrophotometer, providing the precise spectral and colorimetric data needed to characterise the printer's colour response across its full gamut and build an accurate profile that enables colour managed printing with predictable, accurate colour reproduction.
Monitor calibration and profiling similarly benefits from spectrophotometric measurement, with spectrophotometer based calibrators offering greater accuracy and consistency than the alternative colorimeter based devices, particularly for wide gamut displays and displays using non-standard phosphors or LED backlights whose spectral emission characteristics may cause colorimeters to produce inaccurate readings. High end spectrophotometers used for monitor profiling capture the full spectral emission of the display at each measured colour point, enabling accurate colorimetric calculations regardless of the specific spectral characteristics of the display technology.
Beyond colour profiling and calibration, spectrophotometers are used in photographic and printing applications for quality control and process monitoring - verifying the consistency of print output across different production runs, measuring the colour accuracy of proofs against press standards, monitoring the stability of display calibration over time, and ensuring that the colour reproduction of a complete imaging workflow from capture through to output meets defined specifications. In scientific and technical photography, spectrophotometry is used to measure and characterise the spectral properties of light sources, filters, emulsions, and other optical and photographic materials with a precision and completeness that simpler colour measurement methods cannot provide.