I've been noticing different kinds of photocells on street lights and I had a few questions:

1. The big difference between photocells that I've noticed is that some have a clear plastic case, and others have an opaque case with a window for the eye. Why is this? What are the advantages/disadvantages of the two?

2. I read that some photocells have magnetic relays and that others are purely electronic. How do the magnetic relays work? What are the advantages/disadvantages of the two?

3. On one of my photocells, it reads "Time Delay up to 2 Mins". Some photocells I've found online are marketed as having an "Instant On/Off Relay". How do they compare?

Thank you in advance.

I hope to answer some of your questions,I may have missed something,but hope to have covered most things:

1.The clear plastic housing is cheaper to produce than a housing with a window.
  Advantage of clear: Cheaper to purchase,can be more energy efficient as more ambient light can reach the eye and it won't turn the fixture on as early.
  Disadvantage of clear: Eventually the housing may yellow or become cloudy on some models reducing the amount of daylight reaching the eye.

2. The magnetic relays work by a lower voltage allowed through the cadmium sulfide eye,when light hits the eye more power is allowed through to the relay coil thus creating a stronger pull on the contacts opening or closing them.
   Advantage: Reliable,heavier duty.
   Disadvantage: Consume a little more power than thermal or electronic and with instant response may cause and interuption of the light if bright lightning flashes or bright headlights etc.
Electronic PC'c are popular now as they use less power to operate than magnetic,the electronic PC's do have a relay but smaller than the standard electromechanical relay PC and seem to be reliable as I haven't observed very many dayburners.

3. The photocontrols with a delayed response are ones with a thermal bimetalic switch which opens when current going through the cadmium sulfide eye,it warms a small element popping open the contact switch. These are the most common type especially the cheaper brands such as Caster. The thermal type can take up to 2 min the respond,these of course are not effected by lightning.The thermal type switch are most common in thread mounted compact square PC's as the thermal switch takes up very little space.

The electronic PC's have on some models the delay feature too - the delay is there added on purpose to filter out random flashes.
These are often equipped wit other features as well, most popular is lamp EOL detection and shut down: Detect cycling lamp and shut it down, but this feature is designed only only for HPS (as far as i know).