In practice, the Lippmann process was not easy to use. Extremely fine-grained high-resolution photographic emulsions are inherently much less light-sensitive than ordinary emulsions, so long exposure times were required. With a lens of large aperture and a very brightly sunlit subject, a camera exposure of less than one minute was sometimes possible, but exposures measured in minutes were typical. Pure spectral colours reproduced brilliantly, but the ill-defined broad bands of wavelengths reflected by real-world objects could be problematic. The process did not produce colour prints on paper and it proved impossible to make a good duplicate of a Lippmann colour photograph by rephotographing it, so each image was unique. A very shallow-angled prism was usually cemented to the front of the finished plate to deflect unwanted surface reflections, and this made plates of any substantial size impractical. The size of his early photographs was 4 cm by 4 cm, increased later to 6.5 cm by 9 cm. The lighting and viewing arrangement required to see the colours to best effect precluded casual use. Although the special plates and a plate holder with a built-in mercury reservoir were commercially available for a few years c. 1900, even expert users found consistent good results elusive and the process never graduated from being a scientifically elegant laboratory curiosity. It did, however, stimulate interest in the further development of colour photography.