The minimum contrast needed for optimal text readability with additive displays (e.g. AR devices) will depend on the spatial structure of the background and text. Natural scenes and text follow similar spectral patterns. Therefore, natural scenes can mask low contrast text – making it difficult to read. In a set of experiments, we determine the minimum viable contrast for readability on an additive display. Reading performance was assessed with an RSVP task. Text was additively overlaid on algorithmically generated images that mimicked natural scene statistics. When the text to background contrast ratio was below 1.6:1, participants’ reading rates decreased rapidly. At low contrast ratios (<1.4:1), this effect was slightly mitigated for out of focus backgrounds, which reduced power from masking frequencies. Above 1.6:1, reading rate did not significantly increase but subjective quality ratings did. These results help inform the possible requirement for the development of additive display systems for consumer products.