A plastic lens is manufactured in such a manner that a cavity is formed between a casting gasket and upper and lower molds, and plastic monomer as a lens material is filled into the cavity and polymerized.
At that time, the plastic monomer must be filled thereinto in an accurate amount. The following methods are conventionally employed as a method which provides accurate filling of the material.
As a first method, the cubic content of a cavity is previously measured and an amount of a liquid corresponding to the cubic content is filled into the cavity.
As a second method, the liquid level is detected in a non-contact manner using a capacitive sensor so as to accurately control an amount to be filled.
As a third method, after a liquid is filled into a vessel, the liquid overflowing from the vessel is absorbed in vacuum so as to detect the filling of the liquid by making use of a pressure difference in a vacuum circuit between a pressure when a gas is absorbed before the liquid overflows and a pressure when the overflown liquid is absorbed, as shown in Japanese Laid-Open Patent Publication No. Hei 4(1992)-128617.
However, according to the first method, when the inside volumes of cavities have dispersion or when an amount of a liquid to be filled thereinto cannot be accurately controlled, the liquid overflows from the cavity or the cavity is insufficiently filled conversely and bubbles remain in the cavity.
According to the second method, it is difficult to miniaturize the capacitive sensor and it is therefore difficult to obtain a capacitive sensor applicable to a lens making cavity. Moreover, it is inherently difficult to improve the accuracy of the capacitive sensor.
Further, according to the third method, a vessel can be filled with a liquid with a high accuracy regardless of a type of a liquid, a shape of a cavity, and the dispersion of the inside volumes of cavities. On the other hand, in the third method, an absorbing pressure must be accurately adjusted. Further, when a liquid having a high polymerizing speed is absorbed, an absorbing pipe is clogged.