| 摘要 |
PURPOSE:To prevent the deterioration of a reproducing characteristic due to the change of temperature and to obtain a stable performance by making the change ratio of the focal distance of a collimater due to the temperature approximately coincident with the coefficient of thermal expansion of the supporting member of the collimator. CONSTITUTION:A collimator lens 3 consists of a Fresnel lens, the base of the collimator lens 3 is formed with the same material as that forming a supporting member 11 supporting a laser light source 1 and the collimator lens 3, and therefore, they have the same coefficient of thermal expansion. The change ratio of the focal distance due to the temperature of the Fresnel lens is equal to that of the base. Accordingly, even when the supporting member 11 expands or contracts due to the change of temperature to cause the change of the distance of the collimator lens 3 and the laser light source 1, the focal distance of the collimator lens is changed by the same degree. Thus, the laser light source 1 is always kept at the focal position of the collimator lens, parallel light 4 is always kept in complete parallelism, and the excellent reproduction characteristic can be realized against the change of temperature. |
