In recent years, there have been increasing opportunities to photograph samples using a digital camera. Digital cameras using a sensor such as a CCD and a CMOS are more sensitive to brightness changes than are direct observations with human eyes or photographing with silver salt film cameras. Because of this, non-uniformity in illumination, which is not a problem in direct eye observations or photographing with silver salt film cameras, is a significant problem when photographing with digital cameras. Therefore, in illumination devices for microscopes, there has arisen a need to further increase the uniformity of illumination.
A conventional measure against non-uniformity in illumination is using the so-called Keller illumination technique, which theoretically provides a uniform illumination on a sample surface. However, because the light intensity distribution over the angular direction of light emitted by the light source is not uniform, non-uniformity in illumination still occurs. Although uniform illumination without any non-uniformity in illumination can be obtained if the angular distribution of light emitted from the light source can be made uniform, this is difficult to achieve in actuality.
As a means to improve the illumination non-uniformity caused by the angular distribution of light, a conventional method has been used wherein an integrator, such as a fly-eye lens, is used to divide a light flux into many parts; thus each part has a more uniform illumination. However, there has been the difficulty that adopting this method makes the illumination optical system device itself large, thereby increasing the cost.
Japanese Patent Application 2005-215992 proposes a method of correcting a non-uniform illumination distribution using an optical element such as a neutral density (hereinafter ND) filter and a frosted filter. A similar optical element is also disclosed in Japanese Laid Open Patent Application 2006-30535. Each of these prior art examples of correcting a non-uniform illumination distribution has problems. For example, if an ND filter is used, the influence of wavelength characteristics cannot be avoided. In other words, using an ND filter does not satisfy the objective of reproducing accurate color by illumination with white light. On the other hand, if a frosted filter is used, there is the problem such that it cannot be processed to have the exact transmittance as calculated (through a numerical simulation). Namely, there remains the inaccuracy that the transmittance calculated in the design stage cannot be realized.
Moreover, in Japanese Laid Open Patent Application 2006-30535, an illumination distribution is corrected using an optical element having a transmittance distribution that directly complements (i.e., is the inverse of) the intensity distribution of the light source. Thus, this method of correcting a non-uniform illumination distribution does not truly utilize the characteristics of the illumination optical system.