What is it known as: Numeral denominational value is visible under magnifying glass between security thread and latent image

Microlettering is a security feature where tiny letters (often the denomination or 'RBI') are printed between the security thread and latent image. These letters are so small (typically 0.5-0.8mm) that they require a magnifying glass to be read clearly. The other options serve different purposes - watermark shows a portrait when held against light, fluorescence causes glowing under UV light, and optically variable ink changes color when tilted.