Due to their versatile way of life as saprophytes, endophytes, and entomopathogens, fungi of the generaMetarhiziumandBeauveriaare exposed to varying illumination conditions in their natural habitats, which makes a thorough adaptation to light very likely. While the few available studies for these genera support this assumption, research in this field is still in its infancy and the data material restricted to only a few fungal species. Thus, the aim of this work was to explore how light influences growth, conidial production and secondary metabolite formation of two industrial relevant strains ofM. brunneum(MA 43, formerlyM. anisopliae var. anisopliaeBIPESCO 5/F52) andB. brongniartii(BIPESCO 2). To achieve this, we constructed an easily adjustable illumination device for highly standardized photophysiological studies of fungi on Petri dishes, the so-called LIGHT BOX. With the aid of this device,M. brunneumandB. brongniartiiwere grown on S4G or S2G agar at 25 °C for 14 days either in complete darkness or under constant illumination with red light (λpeak= 635 nm), green light (λpeak= 519 nm) or blue light (λpeak= 452 nm). In addition, for each wavelength the effect of different illumination intensities was tested, i.e., intensities of red light ranging from 22.1 ± 0.1 to 136.5 ± 0.3 µW cm-2, green light from 16.5 ± 0.1 to 96.2 ± 0.1 µW cm-2, and blue light from 56.1 ± 0.2 to 188.9 ± 0.6 µW cm-2. Both fungi strongly responded in terms of growth, conidial production, pigmentation and morphology to changes in the wavelength and irradiation intensity. The wavelength-dependent production of the well-known secondary metabolite oosporein which is secreted by the genusBeauveriain particular, was also increased under green and blue light exposure. The established LIGHT BOX system allows not only to optimize conidial production yields with these biotechnologically relevant fungi, but also allows the photobiological exploration of other fungi.Supplementary InformationThe online version contains supplementary material available at 10.1186/s40694-023-00159-w.