The impact of the optical properties on the annual performance of flat plate collectors in a Swedish climate has been estimated with the MINSUN program. The collector parameters were determined with a theoretically based calculation program verified from laboratory measurements. The importance of changes in solar absorptance and thermal emittance of the absorber, the addition of a teflon film or a teflon honeycomb, antireflection treatment of the, cover glazing and combinations of these improvements were investigated. The results show that several improvements can be achieved for solar thermal absorbers. A combined increase in absorptance from 0.95 to 0.97 and a decrease in emittance from 0.10 to 0.05 increase the annual performance with 6.7% at 50 degreesC operating temperature. The increase in performance by installing a teflon film as second glazing was estimated to 5.6% at 50 degreesC. If instead a teflon honeycomb is installed, a twice as high performance increase is obtained, 12.1 %. Antireflection treatment of the cover glazing increases the annual output with 6.5% at 50 degreesC. A combination of absorber improvements together with a teflon honeycomb and an antireflection treated glazing results in a total increase of 24.6% at 50 degreesC. Including external booster reflectors increases the expected annual output at 50 degreesC to 19.9-29.4% depending on reflector material. (C) 2002 Elsevier Science Ltd. All rights reserved.