Well, you don't specifically need an optotransistor; an opto-diode or a light-dependant resistor (LDR) could also be used.
And there are pleny of other light-sensitive devices available - including calibrated measurement sensors.
Which would be most appropriate is impossible to say without further details of the actual requirement


To simply detect the presence or absence of light, a simple LDR, phototransistor, etc and a schmitt trigger would be sufficient to give a binary Light/Dark indication.

To actually measure the light intensity (luminous flux) would, of course, require some sort of analogue-to-digital conversion. As Erik said, there are readily available off-the-shelf direct-to-digital sensors.

Don't forget that "light" covers a whole spectrum - literally!
Different transducers have different spectral response properties - this may or may not be important for this particular requirement...