Photometric titration If we have  $\ce{A + T -&gt; P}$ 
where $\ce{A}$ is the analyte, $\ce{T}$ is the titrant, and $\ce{P}$ is the product,
photometric titration happens only in one condition, that one of the following substances will absorb the electromagnetic radiation. 

When stable complexes formed such as $\ce{Bi}$-EDTA, they will not absorb the radiation (or in small amounts), but when unstable complexes formed such as $\ce{Cu}$-EDTA, they will absorb highly. The same goes also for the acids. Strong acids will absorb much less the electromagnetic radiation than the weak acids. 
The reason for this is that strong acids are completely dissociated.

Why does that indicate the photometric titration ?
photometric titration happens only in one condition, that one of the following substances will absorb the electromagnetic radiation.


Almost ;-)

Regardless whether you monitor a complexometric titration by eye or with the help of a spectrometer, it is a good idea to use an indicator, such as Eriochrome Black T. Then, it doesn't matter, if or how  strong the EDTA complex of the metal absorbs!

Instead, you monitor the absorption of the indicator, that is liberated from the metal-indicator complex upon addition of EDTA.