When it comes to the diameter of any bulb, they are generally always marked in a "T-?" fashion, and the number that is in place of the question mark is the number of 1/8th's of an inch that it is. So the T5 bulb is 5/8", whereas VHO and regular flourescents range from T10 to T12, with VHOs being exclusively T12 I believe, meaning that they are between 1 1/4 to 1 1/2 inches in diameter, and most assuredly 1 1/2" when it comes to the VHO bulbs. One thing to look out for though with VHO bulbs is that there are generally two options when it comes to bulb length in that some are themselves 48" or 72" or whatever, while some are 1/2 to 1 1/2" shorter so that they can fit into a 48" or 72" hood. So, just be careful that you look at the right numbers when picking up bulbs for whatever lighting system you decide to go with.