CN105812661B - A kind of digital camera uniformity correcting method based on light box and gray card - Google Patents

Abstract

The invention discloses a kind of digital camera uniformity correcting method based on light box and gray card.Standard gray card is positioned in light box by this method, translates the image for obtaining standard color card by way of camera is repeatedly shot in image different location vertical optical axis, homogeneity correction coefficient is mathematically obtained finally by difference or fitting.The method of the present invention exchanges the spatially uniform of light source for the time stability of light source, overcome traditional digital cameras uniformity test and correction must be in the limitation using such as integrating sphere uniform source system, the scope of application for considerably increasing test and correcting digital camera imaging uniformity, also avoid as metamerism phenomenon that color filter type integrating sphere light source generates and caused by measurement error.

Description

A digital camera uniformity correction method based on standard light box and gray card

technical field

The invention relates to the fields of optical imaging and digital image processing, in particular to a method for correcting digital camera imaging uniformity based on special uniform light source devices such as a standard light box and a gray card instead of an integrating sphere.

Background technique

At present, digital cameras have been widely used, not only for shooting images to record scenery, but also for imaging color measurement equipment in the field of scientific research. However, the imaging of digital cameras is not perfect, and one of the typical imaging phenomena is the non-uniformity of imaging. Due to the vignetting diaphragm of the lens, the cosine vignetting of the off-axis image point (the illuminance of the off-axis image point decreases with the fourth power of the cosine of the field angle), the non-uniformity of the imaging element (such as CCD or CMOS) itself, etc. Due to the combined effect of various factors, the camera's imaging of an object with uniform brightness will eventually show non-uniform characteristics. This imaging non-uniformity is also often referred to as shading.

ISO 17957 recommends shadow measurement conditions for digital camera imaging: A uniform field target is required that can represent light sources for three common lighting environments, namely, average outdoor daylight, indoor incandescent lighting, and indoor fluorescent lighting. In order to generate the required test conditions, a lighting environment specifically set up for testing shadows is required. The industry usually uses an integrating sphere uniform light source system to provide uniform lighting conditions. On the one hand, such test conditions and methods greatly limit the test occasions for digital camera uniformity test and correction, which brings inconvenience to actual measurement and correction, especially for digital cameras with wide-angle lenses, it is necessary to provide a large enough area Uniform lighting is even more difficult. On the other hand, in order to realize the three typical light sources specified by ISO, currently integrating spheres often use the combination of incandescent lamps and filters to produce different types of light sources. Spectrum causing potential risk of measurement inaccuracy.

Contents of the invention

In order to overcome the inconvenience of using a special uniform light source device in the existing digital camera imaging uniformity testing and calibration method, the present invention provides a method for testing and calibrating the digital camera imaging uniformity using a standard light box and a standard gray card.

The object of the present invention is achieved by the following technical solutions: a digital camera uniformity correction method based on a standard light box and a gray card, the method may further comprise the steps:

(1) Arrange the light box and place the gray card: place the light box in the dark room, place the gray card in the light box, the gray card and the bottom surface of the light box form a fixed angle.

(2) Turn on a light source of the light box to preheat the light source.

(3) Camera position and parameter setting: After the light box light source is preheated, place the digital camera at a distance where the gray card can focus normally. The optical axis of the digital camera is perpendicular to the plane of the gray card. The exposure of the camera to the gray card is correct; the shooting parameters of the digital camera include ISO sensitivity, aperture, shutter speed and focal length.

(4) Camera shooting: Translate and shoot the digital camera on a plane perpendicular to the optical axis, so that the gray cards obtained from N times of shooting are scattered and appear in various positions of the image as much as possible; extract the evenly illuminated area of the gray card in the i-th picture The average value RGB _i of the RGB values of , and the center position q _i of the area, i=1,2,...,N.

(5) Predict and correct the uniformity of the camera: According to the position variable q _i and the image response value RGB _i obtained in step (4), the prediction function mapped from the position variable q _i to the image response value RGB _i is obtained by interpolation or fitting f( ), for any point p in the image P, the uniformity correction coefficient k(p) of the point is obtained:

(6) Change the light source type of the light box, repeat steps (2) to (5), until all types of light sources are tested, and obtain the uniformity correction coefficient of the digital camera for each light source.

The beneficial effects of the present invention are: the present invention uses a standard light box and a standard gray card, adopts the method of shifting the camera multiple times to shoot the same standard color card, and obtains the uniformity correction coefficient of the image through the mathematical method of interpolation and fitting. This method trades the temporal stability of the light source for the spatial uniformity of the light source, overcomes the limitation that the traditional digital camera uniformity test and correction must use a uniform light source system such as an integrating sphere, and greatly increases the scope of application for testing and correcting digital camera imaging uniformity .

Description of drawings

Figure 1 is a flow chart of digital camera imaging uniformity testing and correction;

Fig. 2 is a test experimental device diagram;

Figure 3 is a standard gray card picture taken in the example;

Fig. 4 is the test result comparison chart of integrating sphere method and the method of the present invention, (a) adopts A light source, (b) adopts D65 light source.

Detailed ways

The following takes Nikon digital SLR camera D3x with Nikon AF-S NIKKOR 24-120mm f/4G ED VR lens, GretagMacbeth Spectralight III standard light box and gray card in X-rite ColorChecker Passport as an example to illustrate the testing and calibration of this digital camera Specific steps for imaging uniformity. It should be noted that the present invention is not limited to Nikon digital SLR cameras, as long as digital cameras that can obtain image digital format output are applicable to the present invention, and are not limited to GretagMacbeth standard light boxes or X-rite color cards, as long as they can provide stable light sources and Color cards containing spectrally neutral color patches are all suitable for the present invention.

As shown in Figure 1, a kind of digital camera uniformity correction method based on standard light box and gray card provided by the present invention comprises the following steps:

(1) Lay out the light box and place the gray card: As shown in Figure 2, place the Spectralight III standard light box in a dark room, turn the X-rite ColorChecker Passport color card to the white balance target page (neutral gray page), and place it on the In the light box, the gray card is at a 45 angle to the bottom of the light box for the camera to capture the image.

(2) Select the light source: choose the D65 light source from the Spectralight III standard light box, and turn on the preheating.

(3) Camera position and parameter setting: After the preheating of the light box light source in step (2), place the D3x digital camera at a distance of 2.0m where the gray card can focus normally. The shooting parameters of the camera were fixed, ISO sensitivity was 200, aperture number was 4, shutter speed was 1/80s, and focal length was 50mm, so that the exposure of the digital camera to the gray card was correct (Ev=0).

(4) Camera shooting: translate the camera from left to right and from top to bottom on a plane perpendicular to the optical axis (the theoretical error caused by axial displacement is only fΔr/[r(f+r)]×100%, where r is the object distance, f is the focal length of the camera, Δr is the axial displacement difference, and the unit of Δr is mm. The theoretical error caused by the axial displacement in this embodiment is only Δr/82000×100%), so that the obtained 40 shots Gray cards appear scattered throughout the image as much as possible. Extract the average value of the RGB values of the uniformly illuminated area of the gray card in the i-th picture, denoted as RGB _i , and the center position of the area, denoted as q _i , where i=1,2,...,40.

(5) Predict and correct the uniformity of the camera: according to the position variable qi obtained in step (4) and the image response value RGB _i , i=1,2,...,40, the position variable is obtained by the two-dimensional cubic interpolation method q _i is mapped to the prediction function f(·) of the image response value RGB _i , for any point p in the image P, the uniformity correction coefficient k(p) of this point is obtained:

In the formula, max( ) is the function for finding the maximum value.

(6) Replace the Spectralight III standard light box with light source A, and repeat steps (2) to (5) to obtain the uniformity correction coefficients of the digital camera for various light sources.

The experimental results take the green channel (G channel) as an example, as shown in Figure 4, the experimental results of the method of the present invention are very close to the correction coefficient obtained under the uniform light source system of the integrating sphere.

Claims (1)

1. a kind of digital camera uniformity correcting method based on light box and gray card, which is characterized in that this method include with Lower step：

(1) it arranges lamp box, places gray card：Lamp box is positioned in darkroom, gray card is positioned in lamp box, gray card and lamp box bottom surface At fixed angle；

(2) a kind of light source for opening lamp box, preheats light source；

(3) camera position and parameter setting：It waits for that lamphouse light source preheating finishes, digital camera is positioned over what gray card can normally focus At distance, digital camera optical axis is arranged perpendicular to gray card plane and fixes digital camera acquisition parameter so that digital camera is to ash Exposed card is correct；The digital camera acquisition parameters include ISO sensitivity, aperture, shutter and focal length；

(4) camera is shot：Digital camera is being translated and shot on optical axial plane so that the gray card that n times are shot The each position for appearing in image is spread as far as possible；Extract the average RGB responses in gray card uniform illumination region in the i-th pictures Value RGB_iAnd the center q in the region_i, i=1,2 ..., N；

(5) uniformity of prediction and correcting camera：The location variable q obtained according to step (4)_iWith average RGB responses RGB_i, It is obtained from location variable q by interpolation or fitting_iIt is mapped to average RGB responses RGB_iAnticipation function f (), for figure As any point p in P, the homogeneity correction coefficient k (p) is obtained：

(6) the light source type for replacing lamp box, repeats step (2)~(5), until all kinds test of light source finishes, obtains the number Code camera is directed to the homogeneity correction coefficient of each light source.