Documentación y análisis

Visible light examination and photography

• General illumination sources,color temperature and spectral characteristics, filtration

  • The first requirement for accuracy in photographic documentation is ensuring a correct reproduction of color. In digital cameras this is done by calibrating all hardware components (camera and monitor), as well as by performing white balancing prior to each session, using an 18% graycard.

  • It is good practice to photograph a color chart such as the Gretag-Macbeth ColorChecker, and measure the RGB values on the camera software (white should be around 245; grey around 160; black around 53).

  • When documenting objects one must keep record of the position of the light source in order to repeat the setup when document during and after treatment images. Light sources can be halogen lamps or calibrated fluorescent light boxes.

• Different illumination techniques allow to record different aspects of the object:

  • Normal illumination - provides a homogenous lighting of the object similar to a normal display situation. In a documentation setup it implies having two light sources positioned at 45 degrees towards the object. It allows observing the image, paper tint, toning, and condition. It is achieved by using two projectors equidistant to the object at a 45 degree angle towards the surface. If the projector light is too harsh causing shadows that can alter the perception of the object, diffusers can be placed in front of the light source to soften the light (these can be achieved with any translucent material such as paper tissue).

  • Raking illumination - lights the object from an angle (using one light source) and allows observing planar distortion.

  • Specular illumination - uses a light source at a parallel angle directly at the object surface (commonly by reflection from a white material or by tilting the object towards the light source). It allows observing surface texture and sheen; surface damage such as abrasion, cracking,or silver mirroring.

  • Axial specular - allows recording the negative view of daguerreotypes. This is achieved by placing a glass at a 45° angle over the object: Chen, 2003.

  • Transmitted - uses a light source placed beneath the object. For certain objects on transparent supports, transmitted light may be the best way to document the image content and other techniques may be best to highlight condition issues such as cockling or breakage. Transmitted light can also be useful for documenting condition issues with objects such as ambrotypes where it can highlight a reticulated backing or a colored glass support.

  • Polarized - uses filters on the light sources and camera lens to allow photography of light waves from a specific direction only. This facilitates clear capture of image content from glossy/reflective photographic objects or objects with silver mirroring.

  • Monochromatic.

• Digital photography: resolution; use of digital camera; file format; exposure and contrast control; color control; file storage and backup.

• Close-up photography, photomicrography, and photomicrography.

• Use of the stereomicroscope

• Photography of 3-D objects