Radiometric Corrections for Multispectral Airborne Video Imagery A. Edirlsinghe, G.E. Chapman, and J.P. Louis Abstract images as well as facilitating good image mosaicking applica- Radiometric fidelity of calibrated airborne video imagery is tions. The vignetting of airborne video camera systems has been important if the imagery is to be used for quantitative analysis studied. King (1991; 1992) and Pellikka (1998)have suggested of target surfaces. A four-channel Multispectral Airborne Video some methods for correction of such vignetting. The magnitude System (MAvS) is used at Charles Sturt University (CSU)for a of vignetting-related radiometric distortions varies consider- range of environmenfal and agricultural monitoring applica- ably with operational system settings such as lens aperture. The tions. The radiometric distortions in the MAVSimagery could objective of this paper is to present the methodology and occur because of lens characteristics in the form of vignetting details of development, of a range of correction procedures to effects and optical aberrations. This paper details the control remove vignetting related radiometric distortions from the experiments conducted to detect and quantify such distortions. MAVS imagery, and to assess the accuracy of the results of these The vignetting in particular is shown to be creating a non- procedures. uniform brightness level across the MAVS imagery. The paper then develops efficient procedures to correct the vignetting in Abedom and Radlometrlc Mstoltlons In the MAVS Imagery the MAVS imagery by producing relevant correction coefticients, In practice, there can be many optical distortions arising from templates, and equations. The accumcy of these de-vignetting the camera lens geometry of a video imaging system. Generally, (correction) procedures is shown to be comparable to the ac- only a part of the light emitted by a point on the object plane, cumcy of similar corrections reported elsewhere. An investi- and which falls on the entrance aperture of the lens, emerges gation into the effect of spectrum related vignetting on the from the lens system and falls on a single point in the image MAVS imagery due to spectral filter characteristics found plane. This is true even for an ideally thin and perfectly spheri- negligible distortions that did not warrant corrections. This cal lens, where rays striking the outer portions of the lens focus particular type of vignetting is usually caused by the wave- closer to the lens than the more central rays. This is called length ships in the band-pass window of a filter at large spherical aberration. Chromatic aberration results in light of incident angles. different wavelengths being focused in different planes. Astig- matism arises where there is a different focal length in the verti- Introduction cal and the horizontal planes, and occurs when rays strike the The process of deriving quantitative rather than qualitative lens off center. Light rays striking the lens diagonally may pro- data from calibrated spaceborne or airborne sensors demands duce comet shaped focal spots known as aberration coma. In a high radiometric accuracy in addition to maintaining geomet- some cases the set of focal points corresponding to all the points ric fidelity of the imagery. For these sensor systems to achieve in the object plane do not form a perfectly flat image plane, pro- their full potential, particularly for multi-temporal studies, it is ducing an aberration called curvature of the field. important that radiometric correction procedures such as de- Due to the small MAvS field of view (FOV)(28"), and the use vignetting procedures be developed to improve the quality and of relatively smaller size apertures (fl1.7 to fl2.3) when acquir- consistency of system response. Presently, among remote sens- ing the airborne video imagery, the rays passing through the ing systems, multispectral airborne video systems are widely lenses are all close to the optical axis of the system. Therefore, used as operational remote sensing tools, due to their high spa- the use of the outer portions of the lenses is minimized and, tial and flexible spectral resolutions (Edirisinghe et al., 1999). consequently, spherical aberration, coma, astigmatism, and The four-band (blue, green, red, and near-infrared) Multispec- field curvature effects are minimal. The chromatic aberration tral Airborne Video System (MAVS) at Charles Sturt University within a band is limited to the 25-nm (narrow band) wave- (CSU) has been operationally monitoring land and water targets length difference, However, the interband chromatic aberration for a number of years (McKenzie et al., 1992;Louis et al., 1995). may be significant if the four MAVS spectral bands are not prop- A schematic diagram of the MAVS is shown in Figure 1, and the erly registered in a composite image. This can be minimized by main sensor characteristics of the MAVS (Edirisinghe et al., camera alignment, geometric correction, and digital registra- 1999;Edirisinghe, 1997) are summarized in Table 1. A 12-mm tion (Edirisinghe, 1997).Consequently, the total effects of these focal length lens is used with each of the four MAv~spectral optical aberrations on the radiometry of most of the airborne bands for normal operations. video imagery is negligible. Full radiometric correction significantly reduces image-to- Radiometric distortion is the appearance of some pixels in image brightness variation in airborne multispectral imagery, the video frame image with false brightness values causing a allowing meaningful radiometric comparison among different A. Edirisinghe is with CCMAR, CSIRO Animal Production, PMB Photogrammetric Engineering & Remote Sensing PO, Wembley, WA 6014, Australia ([email protected],au). Vol. 67, No. 8, August 2001, pp. 915-922. G.E. Chapman and J.P. Louis are with the Spatial Analysis 0099-1112/01/670&915$3.00/0 Research Group, Charles Sturt University, Wagga Wagga, NSW, O 2001 American Society for Photogrammetry 2678, Australia. and Remote Sensing PHOTOORAMMETRIC ENGINEERING & REMOTE SENSING August 200I 9s -- I light that passes through the aperture. Light fall-off is a combi- nation of two factors: the cos4 Blaw of illuminance and vignett- ing, where Bis the incident angle (Ray, 1988). The theoretical limit of the cos4 Blaw characterizes the geometrical and optical basis for the reduction of light in the periphery of the imagery according to central projection geometry (Ray, 1988). A simple derivation of the cos4 Blaw for an optical system can be found in Schreiber (1993). Light fall-off in digital imagery can analogously be mod- eled by a cos4 Blaw of illumination reduction from the center to the edge of the image, where Bis the angular deviation of a target pixel from the optical axis of the system. vignetting in an optical system occurs due to light absorp- tion by lens walls and internal shadowing of off-axis light by components of the lens (Slater, 1980). The effect of vignetting is significant for lenses with large apertures. Vignetting is the net effect of the two independent processes known as optical vignetting and mechanical vignetting (Ray, 1988). The cos4 B VGA Vista Switcher reduction of light in the periphery of the imagery is termed nat- ural vignetting (Ray, 1988). Therefore, the total effect of light fall-off can be described by the net effect of the three types of vignetting known as natural vignetting, optical vignetting, and mechanical vignetting. This description allows the term "light fall-off" to be replaced with the term "vignetting" in general. In the remainder of this paper, unless the terms natural, optical, or Figure 1. Schematic diagram of the MAVS. mechanical are used in conjunction with the word vignetting to describe specific cases of vignetting, the word vignetting in general refers to the total effect of light fall-off in the periphery of the imagery. spatially non-uniform response across the sensor FOv. In the Optical vignetting is caused by the reduction of the cross- case of the MAVS imagery, the sources of these radiometric dis- sectional area of an oblique beam traversing the lens in com- tortions are the atmosphere between the target and the sensor, parison to that of an equivalent axial beam. The effect of this is problems in the video signal digitization phase (e.g., low sig- a reduction in image illumination, due to physical length of the nal-to-noise ratio or inadequate sampling frequency rate), sun lens, the position of the aperture stop, and the diameter of the anglelview angle (reflectance) variations, cover-type bright- front and rear elements (Ray, 1988). The magnitude may be ness variation, non-uniform response due to defective sensor estimated by projecting the image of the stop and rear element elements (missing pixels) or image plane misalignment, and in the object space onto the front element. This reduction of vignetting. Generally, the full radiometric calibration of a illumination in the imagery is also known as "cat's-eye effect." remote sensing system addresses the problem of securing the The amount of vignetting depends on the f number in use; for radiometric fidelity of an imaging system. This includes radio- an aperture with high f number, the optical vignetting is mini- metric corrections such as vignetting corrections and bi-direc- mal while, for an aperture with low f number, the effect of opti- tional reflectance variation corrections, as well as establishing cal vignetting may be high. absolute radiometric calibration or building relationships Mechanical vignetting is caused by mechanical features of between radiometric response of the system and actual the lens intruding into the Fov, causing some peripheral dark- reflectancelradiance received by the sensors. This paper ening of the image due to absorption by the lens walls (Ray, addresses the problem of vignetting in the MAVS imagery and its 1988). This can be the rim of a lens hood that is too long or of correction. The radiometric calibration of the airborne video the wrong aspect ratio.