In the last 10 years I have noticed more and more magihim checking STa”M with the primary light source situated behind the parshiyos being checked. This is achieved with use of light tables, light boxes, lamps behind transparent boards, etc, without the use of additional frontal lighting.
Using such lighting is often advantageous, because it brightens the parsha and enhances the clarity of the letters, making them appear more stark and easier to see. It is also helpful in detecting small nifsakim in letters that may otherwise go unnoticed in normal lighting conditions.
However many sofrim may not realize that checking in such a manner also had drawbacks. The most serious one is that it is much harder to detect smaller negiyos , particularly when the negiya is thinner and/ or a little lighter in nature. I have experienced this countless times personally: when my lightbox is on I don’t notice a negiya, yet when it is off, I see it clearly. I encourage anyone to test out this phenomenon themselves and they will see what I mean.
It is without doubt that the advantage - allowing one to see small nifsakim that are not readily visible in normal (frontal) light, becomes a disadvantage when it comes to the detection of negiyos.
I would therefore like to argue the following: We know that a "nifsak shenikkar rak neged hashemesh" (ie when seen only against a light source) is not meakev and does not need to be fixed (of course, one can if they want to, to stop it getting worse, but this is not required). However to overlook a negiya is a searious breach , and can easily invalidate the parsha. So to my thinking, the whole backlight / rear light source practice is very dangerous.IMHO, I think it is only safe to use light tables /light boxes/ lamps behind transparent boards, etc if you either:
1) Check once with the backlight and again with a frontal/ normal light. This will give you the plus of having one hagoha with the clarity and advantages of the backlight. The second hagoha will ensure you don’t miss negiyos.
2) When using the backlight, have an additional lamp of similar strength situated frontally, shining onto the front of the parsha. This won’t contrast and clarify the letters quite as much as using a backlight alone, however it is still quite effective and negiyos are completely visible.
Alternatively raisng the parsha and examining it in a vertical position directly under the light globe, where part of the light shines behind the klaf and part in front, has the same result. (The disadvantage is there is nothing for it to rest on so one has to put it down when making tikkunim. )
I think it’s important to point this out, particularly since the practice is becoming so commonplace. If anyone does not agree with this or has had a different experience to mine, please feel free to comment.