An apparatus for detecting ophthalmic diseases, such as inflammation in a patient's eye, includes a projection system for projecting and focusing a laser beam at a selected spot in the eye. The laser light scattered from the eye is photoelectrically detected through a slit in a mask and converted into an electrical signal which is used to determine the protein concentration in the patient's eye. A light shield member is displaceable in front of the mask slit to selectively block diffused light from the eye or scattered laser light to improve the S/N ratio of the electrical signal.