Light Margin Indicators (LMIs) are used with some barcode types to indicate the extent of the mandatory "quiet zones" on either side of the main bars.

Quiet zones are neccessary for the scanner to detect the start and end of the barcode.

LMIs help to discourage designers from placing other images too close to the bars.

They are not required by UPC-A barcodes as the first and last human readable characters protect the quiet zones on both sides.

EAN-13, ISSN and ISBN barcodes only require one LMI (on the right).