Every printer needs a way of locating the Top of Form (top of the label.) Thermal Barcode Printers have two ways of doing this. Which one you use depends on the type of label you are using.
How the printer locates the Top of Form
- Gap/Notch Media
- Black Line Media (Black Bar, Mark)
- Continuous Media
Most labels have an 1/8th inch gap between each label. In this case the printer uses a Transmissive or Gap sensor. This sensor shines a light at the label while another sensor measures how much light is getting through. More light gets though the gap between labels letting the printer know where the Top of Form is. The same sensor works on labels/tags that have a hole or notch on one side.
The second type of sensor is a Reflective or Mark sensor. It is used for media that has a black line on the back. Instead of shining light through the label it shines light on the bottom and watches how much light is reflected back. When the light hits the black line the light level changes, letting the printer know where the Top of Form is.
Some media does not use either sensor. It is called Continuous, meaning there is no way for the printer to determine top of form. In these cases the sensors are just turned off. The printer just prints when you tell it to without looking for top of form. Receipt paper is an example of a continuous media.