A reflector is a device that reflects the bulbs light at one direction. Grow lights reflectors are usually reflecting the light downward! Basically reflectors is any surface that reflects some amount of light but not any surface can be used with T5 grow lights, because indoor growers need as much efficiency as possible and T5 grow light reflectors usually have close to perfect light reflection. This is usually achieved with materials like aluminium or mylar. Usually the material used in pre-built t5 grow lights is aluminium or mylar and if you have to choose one I suggest the one with the mylar reflector as it reflects around 99 % of the light that shines on it.
How reflectors work?
Basically a reflector is any surface that doesn’t absorb all of the light that shines on it. So if a material absorbs any light it become less efficient as a reflector and that is a bad thing. After knowing the reflectiveness of a surface inexperienced growers could think that it is it and that the material reflectiveness is the only thing that matters. But this is not true because there is also a necessity to reflect this light evenly not creating any hot or cold patches on your plant leafs. So after knowing how reflective a material is and knowing how smooth this material reflects light, grow light manufacturers use the best one and create a parabolic or rectangular shape. These shape dictate how will the light be reflected and how big will the reflective surface be and how intensely the light will shine.
The half circle shape reflectors usually work good for reflecting a light in a more direct and smaller space increasing the light intensity but reducing the spread.
The straight wing reflector however is meant for much wider light reflection and therefore they give more even light spread to a larger distance but they don’t give off that intense light.
So when choosing a T5 reflector you need to think about both these reflector types but in the real world pre-made fixtures have built in reflectors and usually they are chosen as the best for that light fixture. And when talking about single bulb reflectors you can choose any reflector because usually the light is not that intense to see any difference between any reflector design but you can definitely see a difference between a bulb with reflector and without it.
As I mentioned before reflectors give more intense light to your garden and that results in a much greater overall plant growth and yield if you are growing plant that produce some kind of flowers and fruits / vegetables. With these reflectors the reflected light can give up to 3 times more light intensity to your garden because the light is directed down from the top and both sides of the bulb which otherwise would have been reflected only from the room ceiling and walls and as you might know light loses its intensity as the distance it travels increases. So these reflectors not only increase the light intensity but also directs more of it in the direction where your plants are growing.
How to choose your reflector?
Basically there are two options to choose for when thinking about buying your next T5 grow light. The options are :
Buying a premade fixture with reflectors
Buying a reflector separately from the bulb
The pre-built T5 grow light fixtures with reflectors are good because the reflectors have been tested and matched to the specific size and overall fixture design whereas if you buy a fluorescent grow bulb reflector separately you need to match the bulbs and the reflector sizes, and conduct a research of which reflector best matches your necessary intensity and spread requirements. So overall I would advise to buy a reflector combined with a fixture but if you already have a T5 grow light than any reflector would greatly increase the overall plant growth.