Abstract: A passive planar thermo-optical structure for detection of a region of changed temperature on patient's skin is provided. The thermo-optical structure has a layered structure including a transparent film layer, a thermochromic layer, a securing layer, an adhesive layer, and a protective layer. At least one dye in the thermochromic layer is above a temperature threshold range of 31°-37° C., which adopts the first colour and keeps it in a temperature range greater than or equal to 0.6° C. A width of this temperature range, in which the colour is indefinite, is smaller than 0.3° C. A method of diagnosing allergy using the above-structure is provided. The method includes the steps of stimulation, measurement and comparison. After stimulating three regions of patient's skin, an evaluation of extent of histamine reaction is performed. Then, the extent of histamine reaction in the third region is compared to that in the first and second regions.