How to detect the imperfection of a ｂｉｃｏｎｃａｖｅlens?

There are two possible ways:
1. Lens fitting
We should find a plano-convex lens which has been confirmed to be perfect and of the same material with the biconcave lens to fit such that the fitted lens is a palno-concave lens as follows:

圖片參考：http://i117.photobucket.com/albums/o61/billy_hywung/April07/Doraemonapril1.jpg

After fitting, place the fitted lens into carbon disuplhide liquid which has a higher refractive index than the lens and then use the Newton's rings method (as stated in http://hk.knowledge.yahoo.com/question/?qid=7007031501244).
2. Focal point shape inspection
With the setup as follows:

圖片參考：http://i117.photobucket.com/albums/o61/billy_hywung/April07/Doraemonapril2.jpg


Carbon dishlphide liquid has a higher refractive index than the lens.
Thus, the parellel rays will converge instead of diverge after passing through the lens.
The light source should be monochromatic so as to avoid any interference pattern raised from different colours since different colours have a little bit different refractive index for the same material.
Then, the white screen is moved to and fro about the lens for observation of the shape of focal point at various distance. (At the focal length, the point appears as a "dot" instead of a "circle" and thus distance adjustment is necessary to inspect if there is any irregularities found on the point.)
Results may appear as follows:

圖片參考：http://i117.photobucket.com/albums/o61/billy_hywung/April07/Doraemonapril3.jpg


圖片參考：http://i117.photobucket.com/albums/o61/billy_hywung/April07/Doraemonapril4.jpg

Note: For a better observation, the experiment can be carried out in a dark environment.

Firstly, biconcave lens has the following features:
1, prismatic effects towards lens edges,
2, Narrow entrance for light beams,

to detect the above imperfections,
1, compare the off-axis light rays to the near-axis light rays (or compare directly with a reduced lens) according to the image quality and the amount of distortion (such as see whether any rainbow form from white beams, ie, chromatic abberation)
2, using light rays striking towards to the optical centre of the lens, then measure the largest incident angle and compare with non-biconcave.

(if u want the steps on setting up the experiments, please let me know)