For most people, the cost of a true spectrometer is out of reach and so the instrument of choice (and even that isn't that common) is to go with a Solarmeter instrument. I've often warned people in the past about the inherent inaccuracy about the Solarmeters, due to how they actually respond to light and give you a measure (ie. an average over a very wide range of unequally measured wavelengths) and have said they really only give a 'ballpark' measure. Little did I realize how enormous a ballpark could be.....
I've just returned from a workshop on amphibian conservation, and one of the exercises we did was compare a Solarmeter 6.2 to a laboratory spectrometer that measured ONLY the UVB wavelengths responsible for Vitamin D3 photoactivation, and the results might be surprising to people. Under relatively weak artificial lighting (I stress, relatively WEAK), the true reading of UVB was about 30% what a Solarmeter indicated (so if you read 10 uW/cm^2 on a Solarmeter, the true utilizable UVB intensity is about 3-4 uW/cm^2). Under full sunlight the accuracy was even worse, the true reading of UVB intensity is only about 10% the value that a Solarmeter gives (so if a Solarmeter says 200 uW/cm^2, then the true reading is only about 20uW/cm^2). I wish we had the time and means to make something of a respectable calibration curve, but sadly this just wasn't possible in the time we had. Nevertheless, it is a start to getting a better understanding of scenario. So if you rely on your Solarmeter to determine if a bulb really needs changing, then I would say that if you're reading 4 uW/cm^2 or less, then it is likely your animals are not receiving utilizable UVB at all.
I've just returned from a workshop on amphibian conservation, and one of the exercises we did was compare a Solarmeter 6.2 to a laboratory spectrometer that measured ONLY the UVB wavelengths responsible for Vitamin D3 photoactivation, and the results might be surprising to people. Under relatively weak artificial lighting (I stress, relatively WEAK), the true reading of UVB was about 30% what a Solarmeter indicated (so if you read 10 uW/cm^2 on a Solarmeter, the true utilizable UVB intensity is about 3-4 uW/cm^2). Under full sunlight the accuracy was even worse, the true reading of UVB intensity is only about 10% the value that a Solarmeter gives (so if a Solarmeter says 200 uW/cm^2, then the true reading is only about 20uW/cm^2). I wish we had the time and means to make something of a respectable calibration curve, but sadly this just wasn't possible in the time we had. Nevertheless, it is a start to getting a better understanding of scenario. So if you rely on your Solarmeter to determine if a bulb really needs changing, then I would say that if you're reading 4 uW/cm^2 or less, then it is likely your animals are not receiving utilizable UVB at all.