Even illumination

Imaging > Imaging Techniques > Visible light imaging > Even illumination

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Even (or standard) illumination imaging techniques provide a record of the object under uniform light. Although the same principles of object photography apply, these techniques are most suited to record the appearance of the object in a reliable and standardized way, without introducing human subjectivity. Images produced with even illumination methods aim to document accurately the physical characteristics of the object at the time of capture, rather than their aesthetic.

The design of the photographic setup depends on several factors: the scale of the object (large vs. small), its geometry (three-dimensional vs. flat-like), the optical properties of its materials (glossy vs. matte, translucent vs. blocked). The available equipment, the capture environment, and the desired image quality are also variables to consider in choosing the most adequate setup.

Working principle

Schematic illustration of a generic even illumination setup. Illustration: Camilla Perondi; sculpture icon: Marcus DeClarke CC-BY

The goal of the even illumination technique is to create a lighting environment that allows the recording of the object’s color and texture as accurately as possible. Simply put, lighting sources emit visible light that interacts with the object’s surface, and a sensor receives and records the light that is reflected by it. However, several factors come into play when designing the setup; the choice of the lighting source impacts which radiations other than visible light are introduced in the system, and the way in which the object interacts with the emitted radiations changes depending on the object’s geometry and the optical properties of the materials (i.e., transmission, reflection, refraction, scattering). A symmetrical setup with diffused lighting is not always the go-to choice, and quality recommendations are preferred instead of step-by-step instructions.

The Imaging Working Group offers a high-end, flexible workflow for even illumination imaging.

The minimal equipment for this experimental setup includes:

• A camera,
• A tripod or a column with a camera mount,
• A prime lens,
• An IR & UV-cut filter to mount on the lens or in front of the sensor,
• Light sources emitting visible light,
• Light stands or other light mounts,
• Color and greyscale reference charts for color and tonal management.

Additional tools are recommended:

• A neutral grey board for flat-field correction,
• An incident light meter to measure the light intensity across the scene,
• A working station to control the session in tethering mode.

Studio setup

Photographic setup with manual panoramic head and diffused lights. Ph: Camilla Perondi CC BY

Copy stand

A studio copy stand setup. Ph: Luisa Casella.

A copy stand (or repro stand) is a rig used for consistently imaging small-to-medium sized and flat-like objects like books, seals, arrowheads, and textile fragments. In addition to conservation photography, copy stand setups are used in digitization programs as they easily allow for consistent imaging.

In its basic form, a copy stand comprises a column for camera mounting and a platform for object placement. Different models are available on the market, ranging from cost-effective solutions to high-end rigs with a motorized column and built-in light source mounts. Some copy stands have height markers on the column, which can turn useful to note the height of the camera during capture, ensuring consistent before- and after-treatment photography. During digitization, maintaining a consistent camera height also allows for a standard resolution to be established throughout the digitization of multiple objects or pages of a book.

To obtain uniform illumination across a subject’s surface, lamps should be positioned as far from the object as possible, aiming slightly beyond the far edges, not at its center, and with an orientation of about 25-30° from the surface plane. Uniformity of illumination can be assessed with an incident light meter held at the center and all four corners of the object.

A notable limitation of the copy stand is the size of the objects that can be captured due to the constraints imposed by the column height, size of the platform and placement of the radiation sources. Small-to-medium sized and flat-like items (e.g., books, seals, arrowheads, textile fragments) fit best in this setup.

• A studio stand is a column only, which can be positioned over a table or wall mounted.
• A traveler copy stand is foldable and portable.
• A book scanner is a single-purpose copy stand with a V-shaped cradle which ensures that books are correctly supported during digitization.
• A rare book digitization copy stand is a highly adjustable book cradle copy stand that gives maximum support to books.

General recommendations

Background

Neutral gray or black matte background is generally preferable to reduce undesired reflections on the object.

Targets and reference charts

A color target and, if possible, a grayscale chart should be included in the scene.

Lens

Using a lens hood or a shade can help prevent stray light from entering the optical path, which would cause undesired glare in the captured image. Taping black paper to the lens hood is an effective and affordable way of extending the hood, if needed.

Lighting

Using softboxes or umbrellas is recommended whenever high-contrast shadows should be avoided.

Reflectors provide a less diffuse light than softboxes or umbrellas; this light effect can turn useful to record more subtle surface variations.

Barn doors, which are adjustable flaps mounted on some lamp models, can help control the direction of light and prevent stray light from entering the optical path.

For relatively flat objects with pronounced texture (e.g., dresses, creased parchment, bas-reliefs), diffused light sources or a combination of diffused and high-contrast lighting are generally recommended, so as to avoid creating confusing sets of overlapping shadows.

Positioning lights at 45° is a general recommendation that does not always apply. In order to minimize undesired reflections, light sources should be positioned outside of the family of angles. In the studio, marking reference lines with tape or paint on the floor ensures efficiency and standardization.

Using the same lighting setup to record before, during and after treatment allows consistent rendering of the object at different stages.

Camera settings

In conservation photography, the ISO should be set to the maximum value (i.e. the lowest number) allowed by the camera to exploit the sensitivity of the sensor. Medium values of diaphragm aperture (f/N) of the lens are generally the go-to choice to minimize geometric and optical aberrations of the optical path. The shutter speed, together with the light intensity, is commonly the parameter to be adjusted to achieve the desired exposure.

Shooting in RAW format allows for the highest degree of malleability of the images, including the creation of custom color correction profiles.

Some image editing software allow for the control of the camera settings in tethered mode, that is, when the camera is connected to the computer via a tethering cable. This represents an advantage as the images can be viewed on a large screen right after the capture, making it easier for quality inspection in respect to viewing on the camera built-in screen.

Flat-field correction

Flat-fielding is an image processing method that digitally corrects shading and lens cast that can result from lighting inconsistencies or optical artifacts of the lens.

An image for flat-field correction consists in shooting a neutral card or sheet under the same setup used for the object imaging. The card or sheet should cover the entire field of view of the camera, it should be flat, and it should not be reflective.

Challenging materials

Gelatin silver prints that exhibit significant silver mirroring can be a challenge to photograph under normal lighting and produce glare.

In some cases books can contain foldout pages, usually consisting of engravings of images. These need extra support during the imaging process to ensure a level position in relation to the camera sensor.

Further readings

Museum Learning Hub. “Module 3, Technical Workshop 3: Imaging Standards and Logistics in Digitization Projects.” YouTube, July 5, 2021.

• Elizabeth Chang provides a detailed overview of the equipment and space setups, technical imaging standards and how these standards contribute to long term digital preservation plans, as well as tools for ensuring that the digital files have the longest lifespan.