Radiometric Calibration

Radiometric calibration quantifies the amount of light measured by a sensor. It is used to convert sensor acquired digital numbers (DNs) to physical values such as radiance. Radiometric calibration is required to put information acquired by different camera systems on the same scale and it corrects for lens roll-off, dark current, bad pixels, and focal plane defects.

Laboratory Radiometric Calibration

Laboratory Radiometric calibration quantifies the amount of light measured by a sensor. It is used to convert sensor acquired digital numbers (DNs) to physical values such as radiance. Radiometric calibration is required to put information acquired by different camera systems on the same scale and it corrects for lens roll-off, dark current, bad pixels, and focal plane defects.

Flat-Fielding

Individual pixels from an image of a uniformly illuminated, constant target will not have the same DN value. A camera’s lens produces a roll-off in intensity, known as vignetting, as you move away from the center of the focal plane. In addition there are detector-to-detector response variations, “bad” detectors and manufacturing induced artifacts that prevent a camera from measuring a uniform value.

Flat-fielding is a calibration process that removes image roll-off along with other local artifacts so that a uniform target, uniformly illuminated produces a uniform image.

I2R performs radiometric camera calibrations that include flat-fielding using uniformly illuminated integrating spheres with exit ports as large as 40 cm.

Linearity

Accurate radiometry requires known camera response to scene brightness. Many camera systems are designed to have a linear response with light level, exposure setting, solid view angle, and gain (ISO).

Measured camera linearity with light level, exposure setting, solid view angle (related to F#) and/or gain (ISO)

Spectral Response

Spectral Response measures a camera’s ability to detect a signal or respond to a particular wavelength of illumination. Camera response is often provided as a function of wavelength across a given wavelength spectrum. Quantifying a camera’s spectral response enables absolute radiometric calibration.

I2R measures camera spectral response from the ultraviolet (UV) through the shortwave infrared (300 nm – 2,500 nm) using quartz-tungsten-halogen (QTH) lamps, xenon arc lamps or black body sources spectrally filtered with monochromators.

I2R provides customers with:

Peak normalized measured camera spectral response as a function of wavelength in a wavelength region of interest

Absolute Radiometry

Hyperspectral Camera Calibrated with SI-traceable Source

Absolute radiometric calibration is a critical process that establishes a precise relationship between a camera's digital number (DN) values and physical units of radiance. This calibration is essential for quantitative remote sensing applications that rely on accurate radiance or reflectance measurements. Absolute radiometric calibration is achieved by imaging an SI-traceable uniform radiance source and developing a conversion from measured DN to radiance.

Radiometrically calibrated camera systems offer several advantages:

Atmospheric Correction: Enables precise atmospheric corrections in remote sensing data
Vegetation Indices: Facilitates the generation of valid vegetation indices
Time Series Analysis: Supports accurate change detection studies over time
Cross-Sensor Comparisons: Enables reliable comparisons between different sensors
Consistent Color Balance: Ensures uniform color representation across images

I2R provides its customers with:

Radiometric calibration coefficients to convert measured DN values to units of radiance for given exposure, gain, and F# settings

In-flight Radiometric Calibration

I2R uses vicarious calibration to determine the radiometric accuracy of a sensors. Typically, data acquired over spectrally smooth scenes, such as CEOS pseudo-invariant calibration (PIC) sites or Radiometric Calibration Network (RadCalNet) sites. RadCalNet provides quality screened 10-nm resolution surface reflectance and scaled top-of-atmosphere(TOA) reflectance, as well as atmospheric measurements, on a half-hourly basis. The reflectance measurements are compared to data acquired by the sensor to estimate radiometric performance.

Gobabeb, Namibia RadCalNet Site

La Crau, France RadCalNet Site

DESIS comparison to TOA RadCalNet data to assess radiometric performance

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