Héliogravure XII – LED UV lighting

From the days of Henry Fox Talbot, ultraviolet light (UV) is the recommended light source to expose the bichromated carbon tissue through the image positive transparency. Although the Sun was the first light provider, promptly other better stable sources were experimented. Even more, research on the first third of the XXth Century revealed that the bichromated gelatin has a peak of sensitivity located at 365nm, 370nm or even 380nm depending on the references. Using a lamp with an emission spectrum providing this range of wavelengths is the best way to control both exposure and contrast (Fig., 1).

Figure 1. Approximate spectral sensitivity of the bichromated gelatin. Data provided by Mc Graw Co. in a Technical Bulletin by Harry Baskerville (1966).

Until few years ago, the most common light sources used for this purpose were high pressure metal-halide lamps and UV light fluorescent tubes. LED (short for Light Emitting Diode) appeared into scene and have been quickly adopted by practiser of the so called photographic alternative processes demanding the use of an UV light source. Although in this area of lighting development things change rapidly, most available LED UV units, tubes, strips or single chips are offered in the ranges of 365 – 380nm and 395 – 405nm. Both of them are useful for most photochemical photographic processes and also for photopolymer films and plates.

Processes demanding the hardening of bichromated gelatin as Carbon Transfer, Collotype, Oil Print and Photogravure on Copperplate will benefit from the shorter range of wavelengths LED type. LED of 395 – 405nm can also be used, but it seems the tanning is not so hard than the provided for the range of 365 – 380nm. Even with an appropriate exposure, this could cause problems in tissue adhesion on the copperplate, not so differentiated hardened and non-hardened gelatin or the presence of the so called “devils” in the shadow areas of the plate.

In the case of photogravure on copperplate (héliogravure) the advantages provided by LED in the range of 365 – 380nm can be listed as:

    • Spectral emission quite approximate to the bichromated gelatin curve of sensitivity.
    • Low power consumption relative to its useful energy.
    • Virtually exempt of heath dissipation.
    • Virtually immediate maximum output (delay of ≈50ns).
    • Long life expectancy. Theoretically between 10.000 and 50.000h.
    • Quite cheap compared with their counterparts.
    • Simple power supply and electrical wiring available on non specialized suppliers.

After having been using a high pressure metal-halide lamp in the past and changed to a bank of black light fluorescent tubes, have decided to update to now available LED UV light source. The two options beyond ready to use exposure units were LED strips or the recently appeared LED tubes. Having found here in Spain a provider of LED tubes with a very reasonable price (see bottom), this is the chosen option. Tubes are of clear thin transparent glass. The fixture is designed as a fluorescent light tube was in the past, being the power supply integrated into the tube. Then the tube is ready to be connected to the main power. The option with connection only at one end is the chosen from several options available.

Eight tubes of 60cm provide a useful lighting area of approx. 50x55cm, large enough to uniformly illuminate the frame of the vacuum press which is of 40x50cm. They incorporate 55 equally spaced chips per tube with a nominal angle of emission of 130º. Even though this allows for a very uniform illumination on the vacuum frame, just below the bank of LED tubes there is installed a piece of frosted glass of 3mm in thickness. It converts the assembly in a diffused light source as the fluorescent tubes used in the past was. The addition of this frosted glass do not shows a problem in terms of useful available energy because the output provided by LED tubes is higher than that emitted by the former fluorescent tubes.

Doing an empirical comparison between the former fluorescent tubes and the LED, some things can be stated:

    • The bank of six fluorescent tubes had a power consumption of 18w each one, a total of 108w.
    • The bank of eight LED tubes have an individual power consumption of 9w, giving a total consumption of 72w.
    • The bank of fluorescent tubes were used at a distance of 9cm from the glass of the vacuum press, providing a reasonable exposure time of about 5min.
    • The frosted glass diffuser, that is now the actual light source, is situated 26cm above the glass of the vacuum press. The standard exposure time it’s been reduced to 3min.
    • Then, the useful energy provided by the LED tubes is clearly higher than that emitted by the fluorescent.

But things are not always as simple as change the light source and follow exposing, etching and printing beautiful photogravures. After first trials in order to establish a proper exposure time both for the screen and the positive transparency, it was stated the higher useful energy emitted by the LED as has been commented earlier. But the results do not show a reasonable linearity although the printer, the ink and the QTR profile were the used earlier. Indeed, separation of tones from black to mid grey was poor. Conversely, from mid gray to highlights, linearity was almost perfect.

This abnormal behaviour would suggest some kind of misleading in the opacity/transmission properties of the positive transparency with this UV light source, at least for the region of shadows and mid gray. In order to confirm this theory, a test of ink opacity was conducted in the same manner as it was done in the past with the bank of fluorescent tubes.

Figure 2. Calibration pattern provided by QTRIP as a mean to evaluate the ink limit for a given printing media.

The test shown in the Fig., 2, provided as a TIFF file by QTRIP in order to evaluate the ink limits for a given printing media, can be printed on the transparency media used in photogravure, exposed onto a carbon tissue and then transferred to a clean piece of transparency media instead of copper, Pictorico OHP in this case. Fig., 3 and 4 show the transfers after exposing to the former BLB fluorescent tubes and the current LED UV tubes.

Figure 3. Gelatin resist transferred onto Pictorico OHP transparency film. Exposed using BLB fluorescent UV light tubes.

Fig., 3 Shows the relative opacity of the EPSON Ultrachrome inks using BLB fluorescents UV light tubes. Note as effective UV blockers are only the Black, Yellow and Light Black inks. Black is the most opaque to the UV radiation, while Yellow and Light Black shows a similar relative opacity, quite less than Black.

Figure 4. Gelatin resist transferred onto Pictorico OHP transparency film. Exposed using LED UV light tubes.

As can be realized in Fig., 4, the relative opacity of the EPSON Ultrachrome inks is quite different when the light source employed is the LED UV tubes. In this case, Yellow ink is the main blocker, followed by Black and in a quite less amount, by Light Black.

As a consequence of this results, the QTRIP profile employed to print the positive transparencies must be rewritten with a new weighting between the respective ink limits. When this is done, calibrating results come back to the previous linear output. The system is now ready to work in the same way it was with the BLB Fluorescent UV light tubes.

Looking for a possible cause of this different behaviour, a comparison between the respective emission spectrum of the former BLB fluorescents and the current LED UV tubes (Fig., 5), shows some minor differences at the region between 380 and 400nm (violet-blue). As the graphs come from the respective suppliers, the horizontal scale has been corrected to match a common size. Nevertheless, the vertical scale is expressed in both cases as relative to the total output. Being these respective outputs quite different, it is not so easy to compare the above commented differences.

Figure 5. Relative radiant intensity for the BLB UV fluorescent tubes (left) and the new LED UV light tubes (right).

In any case, this experience puts into evidence the need for calibration and even more, the relative useless of transferring absolute data to another user.

LED tubes provider: LED UV tubes employed in this assay are those provided by WëtaLux (https://www.wetalux.com/tubos-t8-color/709-727-tubo-led-t8-aluminio-60-cm-10w-ultravioleta-365nm-transparente-ac-230v-conexion-1-lado.html).

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