1. A method of determining a target brightness value of a target area for an immunoassay apparatus, the method comprising: photographing, with a camera, a subject comprising a surrounding area and the target area to provide an image of the subject to an image sensor of the camera; converting the image to an electrical signal at the image sensor; determining a second-order approximate equation y=ax 2 with x denoting a position value within the subject, y denoting a brightness value depending on x, and ‘a’ denoting a coefficient such that y=ax 2 approximates a light intensity difference that is caused by uneven illumination and/or lens curvature; determining a difference between y and a brightness value of zero point (origin point); correcting y using the difference; determining the target brightness value of the target area according to x and a corrected brightness value y′ given by correcting y, setting, as an origin point (0, 0) of y=ax 2 , according to the maximum or minimum brightness value of the surrounding area.