Partly for my own peace of mind, I decided to trace the story of radioactive watch lume as used by Seiko. This turned out to be quite tricky, especially when trying to pin down exact dates and time periods for the use of different lume varieties. Although my research has led me to some interesting findings, I would not declare it to be complete in its conclusions, nor would I claim it to be the last word in accuracy. Nevertheless, I have decided that there would be sufficient interest among Forum readers for me to write up my research as a Forum topic.
I start the story with World War Two, following the available evidential data from sources and not knowing enough about lume on earlier Seiko watches to write about it here. During World War Two, Seikosha produced a multitude of wristwatches and instruments under Japanese military contracts and we know that radium lume was used on those items where night-time visibility was crucial - such as aviator watches for the purpose of night-time operations. This military period of Seiko lume is co-existent with (and perhaps intimately related to) Japan-based Kenzo Nemoto, who founded his luminous paint processing company in December 1941. During World War 2, he was contracted to supply military aircraft and submarines with lumed gauges and meters, and it may have been his company that provided the lume for Seiko military watches and instruments during the War.
Immediately after the War, to supplement the decline in his business caused by the advent of peacetime, Kenzo Nemoto had the idea of using his luminous paint for civilian clocks and watches, and he apparently even went to the homes of customers where he painted clocks, and subsequently watches also, with his luminous paint. Finally, he approached Seiko with his idea and Nemoto & Co commenced work for Seiko. According to Nemoto & Co (and quoted in Keep the Time Blog, c.2015):
“Kenzo hit on the idea of painting luminous paint on the numerals and hands of watches and clocks to allow one to tell the time even at night. Kenzo took this idea to Seiko, a corporation of watches and clocks, and asked Seiko to ‘let him paint luminous paint on the dial plates of clocks.’ After receiving the dial plates of clocks from Seiko, Kenzo would paint luminous paint on dial plates in a small workshop next to his house. Kenzo started work by delivering clocks painted with luminous paint to Seiko.”
Given the history of lume generally, it is likely that Nemoto’s first foray into the civilian world of lume involved the use of radium. However, it is perhaps possible that he might also have produced a safe photo-luminescent type of lume such as copper-doped zinc sulphide perhaps for use on cheaper watches (and later, as we shall see, the Nemoto company was to revolutionize watch lume by creating the first really effective, safe, and non-ionizing luminous coating). It is not clear just how influential the two wartime atomic bombs on Hiroshima and Nagasaki were in speeding up the search for safer non-radium lume on clocks and watches in Japan after the War, but they probably had some effect in driving Seiko towards finding a suitable alternative to radium - still creating a luminescence that didn’t fade within a few hours, but one that emitted less ionizing radiation. Unfortunately, when it comes to Seiko, we have no firm evidence to guide us as to exactly when radium was abandoned as a lume activator, and we are left with an estimate partly relying on what was happening to lume technology in other parts of the world. I have read that a date of about 1960 marks the end of radium lume at Seiko, though one might push this forwards by some years to err on the side of caution.
In connection with the end of radium lume at Seiko, I note the following item by killsnapz, posted 27 January 2017 on IMeasure et al., 2017:
“The 6119-8110 dials are marked with RAD. The[y] are a very sought after model and very collectible as they were well liked by the troops in Vietnam during the war. I am sure there are other[s] but that one comes to mind.”
Unfortunately, doubts have been cast on the idea that the letters “R”, “T” or “P” printed alongside other numbers at the bottom of the dial on some Seiko watches relate to the lume terms “Radium”, “Tritium”, and “Promethium”, especially for watches made after Seiko had abandoned the use of radium lume. I also note that some Seiko quartz watches from the 1980s are apparently marked for Promethium lume on the dial with the letter “P” in a circle. More research on dial markings at Seiko is needed.
With radium out of the running for health and safety reasons, a replacement was necessary, and here we find a divergence between Seiko in Japan and the bulk of the Swiss watch industry. In Switzerland, a few companies experimented for a while with promethium, but generally lume steadily moved away from the use of radium as an activator, largely in favour of tritium, starting in the early 1960s. In Japan, however, perhaps after a period of experimentation, Seiko preferred to use promethium. Promethium is very much a post-War product; the element was discovered in 1945 (but not announced until 1947), when the isotopes promethium-147 and promethium-149 were first isolated. Promethium-147 is far safer than radium as a lume activator; it still emits ionizing radiation however, especially at the beginning of its life when first applied to a watch. Promethium-147 has a very short half-life of just over two and a half years, which is a distinct disadvantage when using it as a lume activator, yet Seiko was to continue using promethium lume as the norm for an extended period of time right up to about the mid-1990s when photo-luminescent LumiNova/LumiBrite became the main, and then the sole, form of Seiko lume .
The evidence that Seiko relied heavily on promethium lume until some point in the 1990s is strong and worthy of a detailed appraisal. Firstly, we have an important document originally posted by Ikuo Tokunaga of the Seiko company, himself a Seiko icon in his own right. In 1996, he posted the following information (quoted by Spaceview on JoeTritium et al. 2011):
“About the luminous paints of Seiko watches, “Radioactive luminous paint” which uses a radioactive material was abolished five years ago in all watches including diver’s watches. We had switched to non-radioactive “Photo-luminescent paint” with high safety extensively.
The “Radioactive luminous paint” which was being used in Seiko watches before was not “Tritium” but “Promethium = [sup]147[/sup]Pm”. Since the quantity of the radioactive material which can be used for one watch is restricted and radiation energy decreases, in the case of a watch used around 10 years, energy will become weak and luminous paint will not shine.”
Tokunaga also provides an interesting, if somewhat confusing, graph (shown on JoeTritium et al., 2011) plotting the luminous intensity (ncd) of the two different photo-luminescent Seiko LumiBrite applications against legibility limit (min). He also includes a comparison plot of what he calls “old photo-luminescent paint” which once again begs the question of whether Seiko at some time in the past (before and/or contiguous with promethium lume) used a doped form of photo-luminescent lume that emitted no ionizing radiation on certain watches. The graph also indicates the number of elapsed minutes before each of the lume types falls below “legible luminous intensity”.
The second major piece of evidence supporting Seiko’s use of promethium lume, at least into the late 1980s, is given by thorn79 on Watchuseek who years ago worked at Virginia Tech (Virginia State Institute and State University). In July 1988, thorn79 purchased a Seiko Quartz SQ watch that glowed in the dark all the time; subsequently, using a Geiger counter, he discovered that it was radioactive and asked “our radiation safety guy” about it. The latter took the watch back to his lab and used more sensitive equipment to measure it, also writing a report or memorandum on the watch (apparently in August 1988). This memorandum includes the following information:
“Contact reading on the watch crystal = 0.04 mr/hr
2 1/2 inches away from the crystal = background
Contact reading on the back of the watch = 0.0025 mr/hr
The maximum radiation exposures from your watch are: 350 mr/yr from the crystal and 22 mr/yr from the back. Very little exposure is realistic from the crystal since the emissions are absorbed very quickly by air. A small radiation dose would be received by the arm area directly under the watch. This highly localized exposure is very minor compared to existing occupation exposure limitations (5000 mr/yr to the whole body, 3000 mr/yr to the skin, 75000 mr/yr to the extremities). Recommended limits to the general public are 10% of the occupational limits.
Discussions with knowledgable people have indicated that the material is probably pm-147. This isotope emits beta particles and has a half-life of 2.6 years. The emissions detected on the outside of the watch are x-rays caused by bremsstralung.”
A final evidential postscript, unfortunately not dated, comes from JoeTritium (JoeTritium et al, 2011.). He discovered this statement in a Seiko watch manual - the first mention he had come across of promethium-147:
“Your SEIKO Automatic Diver’s Watch uses promethium-147 as the active ingredient in the luminous paint.
The use of radioactive materials such as promethium-147 is strictly regulated by Japanese Law and monitored by the Japanese Science and Technology Agency. SEIKO luminous watches, manufactured in accordance with the law, contain an approved amount of promethium-147 harmless to human beings. The amount of radiation they emit also conforms to the standard specified by the Us Nuclear Regulatory Commission.
^If the watch glass is broken and the luminous paint on the dial and hands is exposed, immediately send the watch to a nearby SEIKO Service Centre, taking care not to touch the luminous paint directly.”
As direct comparison with this notice from Seiko, we have a similar watch manual note from Citizen - an fellow major Japanese watch company competing with Seiko - also relating to dive watches:
“8. Luminous Paint
Being a diver’s watch, luminous paint has been used for this diver’s watch(200M).
200M to facilitate time readability in a dark place. This luminous paint contains a trace quantity of radioactive substance pursuant to ISO safety standard. There is no problem for normal use; however, in the event that the watch glass has been damaged or broken, immediately carry it to the store where you purchased or to an authorized Citizen Dealer.”
This warning notice is debated on the forum thread, and a number of watch models have been encountered with manuals that mention promethium being used as the lume activator (NY0054, NY0023 and NY0040). It seems likely that by the time these models were in production (two apparently having production dates of 2008 and 2010), Citizen had abandoned the use of promethium-147, even on dive watches. According to Sir Les, on the forum thread, Citizen phased out promethium in the late 1980s, and then adopted photo-luminous “Natulite” lume, a similar product to Seiko LumiBrite. There is evidence to suggest that after the demise of radium lume, Citizen followed a path rather similar to that of Seiko, experimenting with and commercially using promethium-147 rather than tritium in some of their watches in the 1970s and 1980s before moving to the new non-ionising photo-luminescent LumiNova-type lume, probably at about the same time as Seiko adopted LumiBrite or shortly after.
Seiko LumiBrite was one of a number of closely related lume products/formulae that derived from the invention by the Japanese Nemoto company of LumiNova in 1993. LumiNova was, or is, a photo-luminescent coating that glows with sufficient intensity for sufficient time for it to effectively be a safe alternative to lume excited by radioactive materials. The key technology within LumiNova that gives it these qualities is the use of strontium aluminate - a hard and long-lasting substance - as the phosphor, doped with a suitable activator such as europium and/or dysprasium. Seiko’s LumiBrite was introduced by Seiko under Nemoto’s patent for LumiNova, and it would seem that the company stole a brief but useful march on other watch manufacturers by acquiring a short-term arrangement with Nemoto for exclusive rights to market the new lume until March 1994. It is not clear if LumiBrite was merely a Seiko name for LumiNova or actually differed in its formulation from the Nemoto product; such a difference would have been marginal.
In connection with a definition of Seiko LumiBrite, the 1996 statement originally posted by Ikuo Tokunaga of Seiko, and already quoted from above, states that:
“The “Photo-luminescent paint ” of Seiko watches is called “LUMIBRITE” (trademark), and is made of SrAl[sub]2[/sub]O[sub]4[/sub] + Eu + Dy. We can use it without any restriction, excluding the above dangerous radioactive materials at all, and it can continue shining several hours and the most important feature of this luminous paint is safety for the users. Although the old photo-luminescent paint disappeared for a short time, in case of “LUMIBRITE”, when it shifts to darkness from a general living environment (conditions which light hits for 10 minutes in the illumination of about 500 lx), it will shine for about 3 to 5 hours by after glow of light in case of general grade “LUMIBRITE”, and in case of the highly efficient “LUMIBRITE” used for diver’s watches and adventure watches etc., it will shine for about 5 to 8 hours in darkness. If even light has hit, long periods of use can be performed repeatedly any number of times, and there is little degradation in this paint, therefore “LUMIBRITE” is an excellent luminous paint for watches.”
I do not intend to discuss at length the full history of LumiNova and Super-LumiNova here because our story of Seiko radioactive lume ends with its full-scale adoption by the company in the form of the nearly identical LumiBrite. However, it is necessary to try and throw light on exactly when LumiBrite became the standard Seiko lume.
In this regard, we have an important document that requires consideration, and the first thing to note is that there were essentially two new varieties of illumination competing directly at the start of the LumiNova period - the photo-luminescent LumiNova-based lumes such as LumiBrite and the Timex electro-luminescent IndiGlo illumination that required the user to press a button to activate the glow. The document we have in evidence (Summerfield, 1994) states that Timex had introduced its IndiGlow technology in May 1993 and that by the end of the year the company was planning to have all its watches luminescent; by the end of October 1993, only 25% of Timex watches were luminescent. As for the qualities of LumiBrite and its adoption by Seiko, Summerfield gives some tantalizing pieces of information, and includes the following statements:
“Seiko Canada and Timex Canada are betting consumer demand for watch dials that light up in the dark will drive sales of an item that prizes fashion over function.
More than 30 models of Seiko’s Lorus brand, and a selection of the Seiko line, now have dials coated with a new luminous substance called Lumibrite.
Timex, which launched the first of its IndiGlo luminescent watches in May 1993, and this August introduced IndiGlo Essential, a new line for women, intends to make all its watch dials luminescent by next year.”
“Jeff Ferguson, Seiko advertising and sales administration manager, says although the target group for the Lumibrite line is consumers aged 15 to 30, there has been a strong response from other market sectors such as seniors and night-duty nurses."
“Jiro Uchida, president of Seiko, says Lumibrite is a virtually permanent, non-radioactive, fluorescent coating that can light up the face of a watch, or just its hands and numbers, for up to five hours after two minutes exposure to sunlight.”
“Lumibrite is a patent of Japanese manufacturer Nemoto. Seiko holds the exclusive rights to market watches using the material until March.
The rollout of Lumibrite is being supported by magazine ads in Campus Canada magazine. The product is also being showcased in Campus Canada Caravan, a program from Canadian Controlled Media Communications that visits colleges and universities across the country.
The ads claim that watches using Lumibrite ‘glow 10 times brighter than ever before with no buttons to push.’
The line is a slap at Timex’ IndiGlo technology, which requires the wearer to push a button to light up the backlit dial.
Seiko has used the technology for a number of years in its alarm clocks. In Jan., the company will introduce it into its Lorus brand with a new product line called Lorus Lite.
As part of its ongoing improvements to IndiGlo, Timex is getting set to introduce a feature called Night Mode, which enables wearers to push any button on the watch to make it light up, rather than fumbling for the right one.”
From Summerfield (1994), it would appear that Seiko had the edge over Timex in the race for safe and effective lume. Electro-luminescence was already being used by Seiko at the time, albeit in alarm clocks rather than watches, and while Timex was involved in scaling down IndiGlo such that it could be used in ladies watches, Seiko Lumibrite, being an applied coating, could already be used in small items and required no buttons to be pressed. It is clear that Summerfield was writing at a time when the use of promethium lume by Seiko was on the cusp of being abandoned, but it is impossible to gauge the exact date when promethium lume was no longer applied to any watch from Seiko. My feeling is that the takeover by LumiBrite was probably quite swift once its clear advantages were realized, and it is likely that by 1996, LumiBrite was the norm as lume on Seiko watches. By that time, it had found its way onto dive and adventure watches, and had moved down into Seiko’s lesser watch brands.
References and Sources Used for the Text of this Topic
Hartov, Oren, “Military Watches of the World: Japan”; Worn & Wound, 14 October, 2020. Online at worn&wound.com/military-watches-of-the-world-japan/
IMeasure et al., “Do vintage Seikos use rhodium and tritium in their dials and hands?”; The Watch Site, thread started 27 January 2017. Online at thewatchsite.com/threads/do-vintage-seikos-use-rhodium-and-tritium-in-their-dials-and-hands.263289/
JoeTritium et al., “Why are Seiko dials so bright?”; thread on Watch Freeks, started 10 May 2011. Online at watchfreeks.com/threads/why-are-seiko-dials-so-bright.16087/
Karrusel et al., ‘“Promethium (Pm) 147”’; thread on the Watch Forum started 16 December 2016, in the Watch Discussion Forum. Online at thewatchforum.co.uk/index.php/topic/107514-promethium-pm-147/&tab=comments#comment-1118801
Keep the Time Blog, “All About Seiko LumiBrite”; article and blog, c.2015. Online at keepthetime.com/blog/seiko-lumibrite/
Keep the Time Blog, “All About Super-Luminova”; article, undated. Online at keepthetime.com/blog/super-luminova/
Love, Zen, “This is the Seiko watch made for Japanese Pilots During WWII”; Gear Patrol, 12 September 2019. Online at gearpatrol.com/watches/a613700/watches-you-should-know-seikosha-tensoku-kamikaze/
Summerfield, Patti, “Seiko launches light line”; Strategy, 31 October 1994. Online at strategyonline.ca/1994/10/31/9701-19941031/
thorn79, “Seiko with Radioactive Hands and Dial”; WatchUSeek Watch Forums, undated but probably October 2020. Online at watchuseek.com/threads/seiko-with-radioactive-hands-and-dial.5248756/
time et al., “So how safe is your lume???”, The Watch Site, August 2010. Online at thewatchsite.com/threads/so-how-safe-is-your-lume.7074/
Wikipedia, “Strontium aluminate”; Wikipedia article, last revised 28 June 2021. Online at en.wikipedia.org/wiki/Strontium_aluminate