1. Field of the Invention
The present invention relates to an original size detecting arrangement and more particularly, to an arrangement for optically detecting sizes of original document for example, in a copying apparatus or the like.
2. Description of the Prior Art
In a copy apparatus having a plurality of paper feeding cassettes, there is provided an original size detecting apparatus for automatically selecting a coyping paper size from various paper sizes corresponding to the size of an original document placed on an original platform of the copying machine.
Conventionally, in general, original size detecting appratuses frequently are of a photo interrupter type wherein the size of the original document is detected by projecting light towards a transparent glass plate which serves as an original document platform form below. It is thereafter detected whether the light projected toward the glass plate is reflected by an original document placed on the glass plate or, after the light reaches an original document cover or a light detecting reflection plate in absence of an original document on the glass plate, is reflected therefrom.
In the conventional original size detection arrangement, it is difficult to accurately discriminate from which part the light is reflected since a stain on the original document cover or reflection plate may effect the reflection of light. In addition, since the light must pass through the glass plate of the original document platform twice, the light tends to be attenuated by the reflections at both surfaces of the glass plate. In particular, in such an arrangement wherein the light is projected toward the glass plate from an inclined direction, the light attenuation is serious, whereby the S/N ratio of a detection signal is lowered.
Accordingly, the conventional original size detecting arrangment inaccurately detects original document size.