When someone is learning to light with continuous lamps, one of the first lights you start with is the quartz. So called because the typical crystal of the bulb is replaced by said mineral in order to withstand the high temperatures reached by the lamp.
Although that same type of bulb is used in many types of lights, it is in video and photography where its full potential is reached. With quartz lights up to 2000W, we can light up large surfaces thanks to the output generated by a single bulb. Combined with an open front (with no front lens like the fresnel lights), some blades, which allow controlling the beam, and a focusing lever to move the bulb back and forth, we get a versatility that few lights give us.
Even though there are lights that by the same power give us many more lumens - that is, much more light - in the power/size relation lays the usefulness of the quartz. We can put them in small or narrow spaces and get a lot of light in an inaccessible interior sets.
Used directly they generate a hard and scattered light, ideal to create contrasting shadows, powerful backlights or even to "emulate" the sun in small spaces (and adjusting the colour temperature, being this warmer than that of our star).
But when we bounce them or filter them with diffusion, it's how the magic is created: it allows us to get a soft light like the one we find in the northern regions where the sky is more cloudy. You can even simulate windows in interiors where there is not even a light entrance.
It is very normal to find such light in 3 light kits like the one we rent in Aclam from the Ianiro brand since, as we also mentioned in the article about lighting with flash, the 3 point light scheme is one of the most used, both for flash as for continuous lighting. A very typical and significant example of this scheme made with quartz is that of an interview, being able to make a general soft light with one of the quartz (both rebound and diffusion) put one as backlight to cut out the interviewee from the background and the last one in angle to add volume and textures to the image.
In the end, the quartz (or red heads as they are colloquially known by their colour) is a light that many of us know and that should never be missing in a relatively complex shooting, because even when we have much more powerful or modern lights, a good quartz can get you out of trouble in many situations.