This invention relates to a light emitting element array and more particularly to such an array used as a light source for a contact type image sensor or an optical image reader.
When image data are read by an optical image reader, a copier or a detector of one kind or another, a beam of light from a source is made incident upon the surface of a document or an object which is being inspected and the reflected beam is converted by a linear image sensor or a charge coupled device (CCD) into an electrical signal which serves as a data input. As the source of light for such a purpose, use is coming to be frequently made of light-emitting diodes (LEDs) because they are superior light sources from the point of view of both reliability and useful lifetime. Since the emitted light energy from a single LED is too small and adequate only for a local exposure, a number of LEDs are frequently arranged in a straight line to form an LED array.
As shown in FIGS. 4, 5 and 6 which are respectively a diagonal view, a side sectional view and a longitudinal sectional view, a prior art LED array typically comprises a plurality of LEDs 4 disposed in a straight line on a base plate 3 such as a printed wiring board. These LEDs are arranged at specified intervals determined by the design and reflective plates 5 are provided between the LEDs 4 such that a target surface 10 can be irradiated uniformly and the number of the LED elements can be reduced. A rod-like lens 6 is firmly attached to a reflective case 2 and supported thereby to be adjacent and parallel to the array of these LEDs 4 such that the light beams from the LED array are linearly focused on the target surface 10.
If the distribution of brightness is measured on the target surface 10 in the direction parallel to the lens 6, however, one obtains a curve as shown in FIG. 7 with a center part of length L.sub.2 where the brightness is more or less uniform. This center part of length L.sub.2 is flanked by end parts of length L.sub.3 where the brightness gradually drops. In general, the length L.sub.2 of the effectively (uniformly) irradiated part at the center where the brightness fluctuations are within a certain specified limit is quite small compared to the total irradiated length. If the length L.sub.2 is desired to be about 216 mm (A4 size), a cylindrical rod-like lens 6 of length L.sub.1 =246 mm is required.
As shown schematically in FIG. 7 and more in detail in FIG. 8, the rod-like lens 6 used in a prior art light emitting element array, with which a brightness distribution as shown in FIG. 7 is obtained, has flat end surfaces 62 formed perpendicularly to its longitudinal direction. Thus, if the diameter of the rod-like lens 6 is 4.5mm, for example, the beams of light emitted from a point 11 which is 1 mm away from its surface and 3 mm longitudinally and internally displaced from the end surface 62 are refracted by the lens 6 as shown in FIG. 8 and scattered in the outwardly direction from the end surface 62. This explains the behavior of the curve in the ineffectively irradiated parts of length L.sub.3 illustrated in FIG. 7.
To require a long lens means that the external dimension of the light emitting diode array which uses it must itself be large. This presents a serious problem if it is desired, for example, to provide a smaller and lighter image reader.