40 mm image intensifiers

Introduction

ProxiVision now offers large area image intensifiers with 40 mm active diameter. Check here the advantages and uses of large-sized intensifiers.

Most efficient coupling to large area CCD

According to Liouville's theorem any optical demagnification (either by taper or by lens) is accompanied by a reduction of luminosity (N_φ m^-2 s^-1). This applies to any stage of the optical chain: on the input side it applies to the projection of the image onto the image intensifier, the same holds true for the image transfer from the image intensifier onto the focal plane array (CCD or CMOS).

[The only (expensive!) exception from that law is the use of a demagnifying inverter, which on the other hand is restricted to fiber optics as an input substrate, thus limiting quantum efficiency and image quality.]

On the input side this effect is however more critical because any loss of input light will result in a loss of signal, whereas at the output it will affect only the signal intensity.

Therefore the performance of an optical sensor can be optimized in many cases by choosing a large area sensor with a matching image intensifier in front.

With the noteable exception of night vision goggles, nowadays virtually all image intensifiers are combined to a focal plane array (CCD or CMOS). With ever increasing number of pixels, large area CCDs are of growing popularity. In particular scientific applications frequently tend to demand sensors larger than 25 mm. In such cases a 40 mm intensifier is the best choice because it allows to use the full active area without image minification, which would be accompanied by undesired side-effects, most noteably distortion and reduced transmission.

Improved light collection with large active area

Not surprisingly, applications that require an image intensifier are typically light-starving. Thus any loss of light on the input side is to be avoided. The projection of the object onto the active device is accompanied with losses proportional to the squared aspect ratio. Thus a 40 mm intensifier will get 2.5 times more light compared to a 25 mm one.

Faster acquisition upon direct contact with sample

One important usage of large intensifiers is the image acquisition in direct contact with self-emitting samples. The two most important fields of application are scintigraphy and X-ray imaging. As no lens can be used, the important parameter is the area directly covered because the acquisition time can be significantly reduced upon usage of a lare-sized intensifier.

Compared to EMCCD: larger active area → lower requirement for optical reduction (i.e. lower losses)

In recent years, technologies competing with image intensifiers have been developed for low-light applications. The most popular of these is the EMCCD. Currently these sensors are manufactured mostly in ½" or 2/3" size. As explained above, the reduction of the image size strongly affects the light collection from the object. Therefore a large image intensifier is intrinsically more light-efficient than any kind of smaller sensor.

Advantages

The common applications for 40 mm intensifiers typically require photocathodes for the visible spectrum, mostly for wavelengths below 600 nm. So S20 and Bialkali photocathodes are the most popular choices and they are well established at ProxiVision.

Indeed most short-gating applications are covered with a common S20 photocathode. The same holds true for situations where the intensifier ”looks“ at a phosphor screen (e.g. in X-ray imaging). Furthermore S20 is more robust a photocathode than S25. For these reasons ProxiVison recommends the usage of S20 photocathode.

All useful configurations are ready for production:

FO or quartz input window
Green / blue photocathode
Single / double MCP
Glas / FO output window
Ring power supply available

Typical specification

	single MCP	double MCP
photocathode	S20		(Na₂KSb)Cs₂
sensitivity	50		mA/W @450 nm
useful diameter	40		mm
outer diameter	70		mm
outer length	≤22		mm
resolution	>30	>25	lp/mm
gain	>2⋅10⁴	>1⋅10⁶	W/W @500 nm
luminous non-uniformity	10	15	% st.dev. / mean
phosphor screen	P43
input window	fiber optic
output window	fiber optic
electronic gain	>1⋅10³	>1⋅10⁶	el/el
EBI	<2⋅10^-7		lx
screen voltage	6		kV
gating	10 - DC		ns

Spectral sensitivity

The sensitivities shown above refer to non-gateable photocathodes on a quartz substrate.

Blemish specification

Definition of zones

zone	40 mm I2
1	8.4 mm x 8.4 mm
2	21.0 mm x 21.0 mm
3	28.0 mm x 28.0 mm

Number of allowed blemishes

	max. blemish size
	in zone 1		in zone 2		in zone 3
Image intensifier types*	75 µm	150 µm	75 µm	150 µm	75 µm	150 µm
BV 4063, BV 4083	minimal	4	minimal	6	minimal	15
BV 4063 -V, BV 4083 -V	minimal	4	minimal	6	minimal	15

*Comments

The sum of all blemish areas must not exceed 1% of the zone area
The size of a long blemish is considered equal to that of circular-shaped blemish with same area
Blemishes with a diameter larger than 150 µm are not allowed
Bright spots visible with a microscope and dark adapted eyes are not allowed at the recommended intensifier voltages inside the useful area of 40 mm