My plants go into the unheated cold frame any time the weather is sunny and over 50 degrees. The grow lights are a poor substitute for natural sunlight, so mine get a dose whenever it is available. On those cooler days, perhaps the incandescent light is placed inside the cold frame to help boost the temperature a bit.
If it is heated, I guess that a cold frame is no longer a cold frame. Perhaps a miniature greenhouse? Anyway, I expect great results from the south facing heated 'hot' frame that I'll construct next year. It will be about six feet long by four feet wide and will be about three feet tall. Three to four incandescent bulbs will be placed on a thermostat receptacle such that they light on demand, probably will set two at 70 degrees and another two at 40 degrees, to come on as a second stage if the first two bulbs are not getting the job done. In the coldest part of the winter, will likely set plants on large rectangular storage boxes filled with water. The water will absorb lots of heat during the day and will slowly release the heat in the evening, giving a constant source of gentle bottom heat during the nights. I'll provide photos of the project from start to finish.
Eclectic gardening style, drawing from 45 years of interest and experience. Mostly plant in raised beds and containers primarily using intensive gardening techniques.