I have a suggestion for your entire sentence:
"If the light from the lighting units, for example fluorescent lights, reflects off the subject surface, the item that is the object of your photograph will, in turn, show up excessively illuminated on the picture."
See http://www.tpub.com/content/photography/14209/css/14209_174....
for another English example sentece of "reflection off the subject surface".

In this context, 写り込み　is indeed "unwanted", but there are some situations, when a photographer is going for a special effect of the light (and thus/or an object) reflecting off from an other object (i.e. when using mirrors, or photographing the reflection of the clouds in the windows of a skyscraper). I believe that is also called 写り込み　in Japanese, here is an example picture of 写り込ませる:
http://trynext.com/travel/070511-1100.php (scroll down to see the picture (http://trynext.com/photo/md/070511-130110.jpg) )
Good luck with the rest of the project!