Custom made whitening trays are made from an impression of the teeth and a thermo tray is produced which works by holding the whitening gel against the teeth for short periods of time over the course of 10-14 days initially then once a week to maintain your whiteness.
The thermo trays are so thin and fit so well that you can go about your normal routine without noticing they are there.
Custom trays perfectly compliment LED teeth whitening as with an LED treatment you can expect to lighten teeth by anywhere between 8-12 shades in a sitting and then follow up with custom made trays to move up the shade guide as well as maintain that whiteness.
The whiteness that can be achieved from trays is a lot deeper and truer and more teeth can be whitened with trays since with LED whitening usually treats only the front 8 teeth.
Generally the incisors are a much darker shade than the rest of the teeth and being denser the yellowing is much harder to shift and causes an uneven shade. With the trays you can target these teeth to gean an even whiteness