Fluorescence emission intensity from a culture of bovine pulmonary artery endothelial cells stained with phalloidin conjugated to Alexa Fluor 633, which targets the intracellular cytoskeletal F-actin network. The visible light absorption maximum of Alexa Fluor 633 is 621 nanometers and the emission maximum occurs at 639 nanometers. The image was captured in 12-bit grayscale mode, but subsequently pseudocolored by converting to the RGB color palette and setting the green and blue channels to zero (to produce a red image). Note the bright fluorescence intensity exhibited by the filamentous actin network, which extends throughout the cytoplasm.