(a) Single stranded DNA under study, labeled at the 5’-end with the organic dye ‘Atto590’. (b) Confocal and wide-field microscope. The filter configuration can be changed in approx. 1 s. WF = wide-field configuration; APD = avalanche photo diode; LP-filter = long-pass filter; CCD = charged coupled device, camera; (c) Laser emission and filter function of the commercial long pass filter. OD = optical density; (d) Spectrum of the sodium reference cell scanning over the D2 resonance frequency. The purple line shows the Doppler-free difference laser lock signal. The blue line shows the measured absorption spectrum of the atomic vapor cell at 200°C. The block band is approx. 6 GHz broad and shows an optical rejection of more than 6 orders of magnitude (measured). The calculated optical density is much higher (OD 100).