In order to achieve a discharge lamp suited to operate under reduced nominal power of e.g. 20-30 W, a lamp is proposed with two electrodes (24) arranged at a distance in a discharge vessel (20, 120) for generating an arc discharge. The discharge vessel (20, 120) has a filling with a substantially free of mercury and comprises a metal halide and a rare gas. The lamp (10, 110) further comprises an outer bulb (18) arranged around the discharge vessel at a distance (d2). The outer bulb (18) is sealed and has a gas filling of a thermal conductivity (λ). The inner diameter (d1) of the discharge vessel is preferably in a range from 2-2.7 mm. The wall thickness (w1) is in a range from 1.4-2 mm. A heat transition coefficient (λ/d2) is calculated as thermal conductivity (λ) at 800° C. of the outer bulb filling divided by the distance (d2). The so-defined heat 10 transition coefficient is below 150 W/(m2K).