All the 19th century experiments that desired monochromatic light, including those that have characterized the photoelectric effect, used dispersive prisms, which separated the light from the Sun or from a candle into its monochromatic components. These are simple components, easily available.

This allowed experiments where the frequency of light was varied continuously, by rotating the prism.

Moreover, already during the first half of the 19th century, it became known that using gas-discharge lamps with various gases or by heating certain substances in a flame you can obtain monochromatic light corresponding to certain spectral lines specific to each substance. This allowed experiments where the wavelength of the light used in them was known with high accuracy.

Already in 1827, Jacques Babinet proposed the replacement of the platinum meter standard with the wavelength of some spectral line, as the base for the unit of length. This proposal has been developed and refined later by Maxwell, in 1870, who proposed to use both the wavelength and the period of some spectral line for the units of length and time. The proposal of Babinet has been adopted in SI in 1960, 133 years later, while the proposal of Maxwell has been adopted in SI in 1983, 113 years later.

So there were no serious difficulties in the 19th century for using monochromatic light. The most important difficulty was that their sources of monochromatic light had very low intensities, in comparison with the lasers that are available today. The low intensity problem was aggravated when coherent light was needed, as that could be obtained only by splitting the already weak light beam that was available. Lasers also provide coherent light, not only light with high intensity, thus they greatly simplify experiments.